By "infection that needed antibiotics", are you referring to a respiratory infection? (aka "RI")
One thing to keep in mind, whether for snakes or humans, is that a course of antibiotics destroys not only the bad bacteria but the good bacteria that promotes
digestion in the gut. Many people take probiotics after a course of antibiotics, or they eat foods such as yogurt & sauerkraut to replenish their beneficial gut flora.
Snakes aren't going to eat those things, but you can get probiotics made for reptiles (such as Bene-Bac) to add to their food or dose them with orally. That may
help? The fuzzy you gave him a few days ago should have been better digested by now, not thrown up. (but I'd also avoid bathing a snake after feeding him-
why does the vet want you to bathe him all the time?)
I'm glad you sent in a stool sample, there may be more going on here than just poor digestion from antibiotics. His "warm end" (of cage) sounds a bit low also,
and snakes cannot digest without enough warmth. He needs a basking temperature of around 88-93*. And if you're running A/C like most people this time of year, his cage temps might be lower than you think. (is his cage in a drafty location? are you sure your temp readings are accurate?)
Does he have hides in his cage on warm & cool sides? Where does he spend his time? Snakes often choose "security" (like a hide) over the proper temperatures (warmth needed to digest), that's why we generally recommend using at least 2 hides, one on the cool end & one on the warm end.