I m so sorry your boy is dealing with this.
He may have been allergic to the fleabites or the Advantage or both.
These can be hot spots, pyoderma or impetigo if there are any sores.Please compare to this link:
What you can do is go to your local pharmacy and get some Burrows solution. This is drying agent. Many time you will find it in a name brand called Domeboro. Just ask the pharmacist what brand names they use. This is used for hot spots. It will help fight infection and decrease itchiness. Clip the hair away in the areas as well before using the drying solution.
Use an antiseptic shampoo called Chlorhexiderm. This in combination with the burrow's solution, will heal hot spots and relieve dryness and itching.
"Shampoos: While degreasing shampoos such as the benzoyl peroxide (oxydex®,pyoben®) and sulfur/salicylate (sebolyte®, sebolux®) shampoos will help remove the skin oils feeding the yeast, there are shampoos that are specifically anti-yeast. We prefer the 4% Chlorhexidine shampoo called Chlorhexiderm Max as it both strips skin oil and kills yeast; however, other anti-yeast products include Selsun Blue, Miconazole shampoo, Nizoral shampoo, and more. The pet must be bathed twice a week to start and the shampoo requires a 15 minute contact time (meaning do not rinse the lather for 15 minutes). "
People food will not hurt him as long as there is no onion or garlic in it. Try eliminating all Corn and gluten for a week and see if the itching stops.
Give him a daily vitamin suppliment with omega 3 fatty acids.
Dogs are carnivores and do not derive much beneft from grains or carbs.Some food is loaded with corn and gluten which can promote allergies and the skin is aways where you see it on a dog.
The product is Sasco aloe-lyptus cream http://sasco.qcommerce.com/
All you have to do is put down some Diatomaceous earth in your yard and a few pots in the house and you and your baby will never see another flea, fly, worm, etc.
I am in Florida where fleas/bugs/worms are active 24/7/365 and it works like a charm for me.
I hope I have covered every possibility.
Try the above treatments and see if it resolves this.
I hope it does and your baby (and you) get relief very soon.Please let me know how he is doing.
I will always be here for both of you.
Warmest best wishes,