My dog has a large irritated skin area. Is it flea bites or allergy?

Poor guy. This does not appear to be flea bites becsuse the area is too large and also is infected. This can however be self trauma which is secondary to a flea allergy. If you see flea dirt (small black specks on the haircoat) or if Tango isn't on a flea preventative, this is a good time to get him on one. I would recommend a good quality flea prevention such as Bravecto, Nexgard, Simparica or Vectra/Firstshield. Then I would recommend getting this skin infection treated through your vet. He needs prescription antibiotics to clear this up. Until you can get him in to be seen, you can try cleansing the area with Betadine diluted 50:50 with warm water and then just leaving it to dry. Don't cover the area as this can lead to further infection. Hydrogen peroxide should also not be used on wounds like this or it could actually lead to further destruction of healthy tissue.
I hope Tango gets this resolved soon, and best of luck treating for any underlying allergies he may have that could have caused this. Best wishes and take care.