Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your fellow is scratching himself raw, even though he is on flea medication.
Since this is such a sudden change for him we need to consider that he may have another parasite that Trifexis doesn't cover, such as Sarcoptes or Demodex mites. With the sort of intense itching he is doing I would highly recommend that he be tested (skin scraping) to look for Sarcoptes mites.
The other possibility is an environmental allergy, or food allergies.
He is a bit old to develop food allergies, we normally see those in young dogs, But inhaled allergies are possible.
You can use a combination of antihistamines and high doses of omega-3 fatty acids to help with the symptoms of inhaled allergies, and they also help with the symptoms of flea allergy. In combination fatty acids and antihistamines work synergistically, much better than either one alone. Symptoms that worsen seasonally are often related to inhaled allergens.
You can try:
1)Benadryl (diphenhydramine only don't use the combination products with acetaminophen or decongestants as they can be toxic for dogs) at 1mg to 2mg per pound of body weight or one 25mg capsule per 15 to 25 pounds of body weight orally every 8 hours.
OR 2)Claritin (loratidine) at 5mg per 25 pounds of body weight once or twice daily.
OR 3)Hydroxyzine at 1mg per pound of body weight orally every 8 hours.
OR 4) Chlorpheniramine at 4mg to 8mg per dog once or twice daily.
OR 5) Zyrtec (Cetirizine hydrochloride) at 1/2 mg per pound of body weight orally every 24 hours. That would be one 10mg tablet per 20 pounds of body weight. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because dogs cannot tolerate decongestants.
Some dogs do better on one antihistamine rather than another. Give the one you pick a week trial and if it isn't working try another. Be aware antihistamines can cause sleepiness or hyperactivity in some dogs. These side effects do wear off with repeated use.
Omega-3 fatty acids are fish oil products. 3V by DVM or Derm Caps ES are good brand name products. Use the high end of the dosing schedule for your pup's weight. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give him 20mg of EPA per pound of body weight per day. For example an 8 pound dog could take 160mg of EPA per day.
Cool water baths with an oatmeal shampoo or chlorhexiderm shampoo (which is antibacterial and antifungal) and a conditioner with an antihistamine may help. The water rinses off allergens and the cool temperature soothes itchy skin. With all of his scratching a secondary skin infection is possible, and those can make him itchy too. Do not bathe your fellow for 2 to 3 days before or after applying topical flea control products or the bath will interfere with the product's efficacy.
Food allergy is possible with him as well if he seems to be itchy all year round. Dogs can develop allergies to any protein or carbohydrate so even if he is only fed one thing that can be what he is allergic to. Dogs with food allergies tend to lick and scratch their paws, face and ears the most, but any of the "allergy reactive areas" can be affected.
Make sure that the food that you put him on is a true hypoallergenic diet. The trouble with "limited ingredient" or "low allergy" pet store brands is that the same machinery is used on multiple lots of food without sterilization cleaning in between. So for example even if a food says it has salmon and rice if the previous batch had beef and corn then you will get traces of those ingredients in your bag of food. Not a big deal if your dog isn't allergic but a waste of money thinking that the food was hypoallergenic and not good for your dog if those happen to be allergens for your dog. The veterinary brand true hypoallergenic foods are more expensive because it isn't cheap to thoroughly remove all traces of a previous food mixture from the machines used to process food.
Generally what I recommend is trying to clear the skin and then adding one food item (chicken, beef, corn wheat etc) every month to see what they react to. Then we can find a regular food to try. As far as permanent diets I do tend to stick with Purina Pro Plan brands or Nature's Recipe as I find those rarely if ever have cross contamination. Purina Pro Plan Turkey and Barley or Nature's Recipe Vegetarian or Venison are pretty good products. I know that this isn't easy from personal experience (my dog is allergic to wheat) and it is time consuming, but worth it.
If you choose to try testing/treating him for a food allergy I recommend that you try a true hypoallergenic diet like Hills z/d or Purina Veterinary Diets HA. No treats, flavored medication or bones while on the diet and it must be used for a least 12 to 16 weeks to see the full effects. You should see some improvement in 6 to 8 weeks.
Cortisone ointment applied to the very itchy places (as long as they aren't raw) twice daily may help as well if needed. Some sprays have alcohol which can be painful on raw skin and lead to more inflammation. I don't like oral steroids but topical directed at the problem areas can be very helpful and are less likely to cause side effects.
Another option if the antihistamines and omega 3's aren't enough is a product called Atopica. It suppresses the immune system a bit so it decreases allergic symptoms but it doesn't have as many harmful side effects as systemic steroids. Another option is a brand new drug called Apoquel (generic name oclacitinib) which interferes with the allergic pathway. It works very quickly to stop the symptoms of an allergy. Most dogs are reported to be much more comfortable in a day or so. Unfortunately it is on backorder now due to overwhelming demand, but it is something to keep in mind for the future if he continues to have trouble.
If you are interested discuss these medications with your veterinarian as they are prescription products.
Another option if you are interested, is trying immunotherapy. Your fellow would need to be tested to determine exactly what he is allergic to, and then he is given small amounts of the allergen to build up his tolerance to it, increasing the amount of allergen in the injection incrementally so that his immune system no longer responds to it. This isn't a quick fix, it takes time to slowly build up their tolerance and as he develops new allergens things may need to be added, but it is an option.
If it's been a while since his last exam parasites like cheyletiella, demodex or sarcoptes mites should be looked for by your veterinarian as well if he isn't improving as they can lead to very itchy skin.
Please let me know if you have any further questions.