Yes, that sure sounds like DH. With you and so many other relatives having celiac disease, I think you can be pretty confident that is what it is.
You can always have him tested with
Enterolab, even after he starts the gluten-free diet.
If you could get him to see a dermatologist really soon to do a skin biopsy (beside an active blister), that would be good. If he has an official diagnosis for DH, he will have his celiac disease diagnosis.
If it takes weeks or even months to get in to see a dermatologist, I would just put him on the gluten-free diet. Especially with him having the genes, there can really be no doubt that he has DH/celiac disease.