did the vet do blood & urine work? did they check for thyroid issues?
Has he had his anal glands look at recently perhaps they need to be expressed, possibly infected?
Any abcess in that area perhaps from a bite from the puppy?
Avoid biting when a simple growl will do
The Spirit Lives As Long As Someone Who Lives Remembers You - Navaho Saying
Vindication ~ For all those pets who became sick or lost their lives from tainted pet food