I've been seeing top reptile vets recommending oral baytril lately, as there is less risk of side effects. This makes sense to me with turtles and omnivorous lizards who eat often, but I'm not sure how I feel about it in snakes. Need to read more & think more about it.
Did the vet do a culture?
Bumping up the basking spot temp by 10˚F will help boost his immune system.