Hello and thank you for asking your question on just answer. My name isXXXXX will try to help.
Hair loss, itching, skin inflammation, and redness around the rear end is usually due to food allergies. Food allergies can develop over time. The most common food allergens are beef, chicken, lamb, corn, wheat, and soy. To determine if he has food allergies, he needs to be put on a prescription, completely hypoallergenic diet (called hills z/d) for 6-8 weeks. You will need to get this from your veterinarian. You can not do the food allergy test with an over the counter diet because they are not completely hypo allergenic. If the diet trial resolves his skin issues, then you know it is food allergies.
To help him in the mean time, you can give him benadryl. The dose is 1 mg per pound every 12 hours. For example, a 50 pound dog would get 50 mg of benadryl every 12 hours.
You can also apply hydrocortisone cream, to the affected areas 3-4 times a day. This will help with itching and inflammation.
The other cause of dermatitis over the lower back is flea allergy dermatitis. This is when a pet is allergic to flea saliva. Then one bite is all it takes for the pet to have a big allergic reaction. I know you said you use advantix flea control. If you bathe him frequently (1-3 times a week), you may have to apply it every 2-3 weeks because it washes off.
I hope this helps.