I'd start with adding LOTS of probiotics along with a digestive enzyme supplement. All those antibiotics are good to kill the bacteria, but will wreak havoc on the digestive and immune system.
Dogzymes has some good options.
Then... find a food that is NOT chicken based (that's a common mastiff trigger). Maybe go with a limited ingredient version, like Lamb & Rice or one of the Fish/Salmon based foods.
Give it 4-8 weeks and see how it goes. It can take a while for things to clear the system.
I'd also add in some Omega-3's; either salmon or fish oil, or feed some tinned sardines (low-sodium or rinse to remove excess salt; packed in water or olive oil not soybean oil), at least once a week.
If you really want to do an elimination diet, the easiest way would be to feed a home prepared diet. Raw or cooked, at least you'd know exactly what was going in the dish. It's hard to do elimination diets with a purchased food. Too many variables.
You might also need to consider environmental allergies.
Any products with fragrance can be a trigger (your shampoo, deodorant, any fabreeze-type sprays, laundry detergents, essential oils, etc). Anything sprayed on the lawn? Flea meds?
Can you track it to a season? Something environmental?
If he's 10 yrs old... how long has the allergy/sores been an issue? Can you track it to any changes in ANYTHING? New person/pet in the house? New routines? New neighbors?
Some food producers will change recipes without notice, which can trigger things, too.
