Agreed. Maybe have someone by to take pictures while you feed and give him meds. That way you can keep everyone updated without putting the extra stress of another handling session on him. The antibiotics are going to take some time to kick in; hopefully he will make it long enough for them to help him. That blister looks like something that's going to look worse before it looks better.

Are you sure he isn't drinking? If he isn't showing obvious symptoms of dehydration it's probably safe to assume he's drinking enough, just not while you're around. If he is showing symptoms of dehydration, you can try gently lowering his chin into his water dish. I've had success doing this with several colubrids that were too ill/weak to bother going to the water dish. Don't push it though; if he tries to move away from the water it's his way of saying he doesn't want a drink. Also, idk if you're still soaking, but that's just putting more stress on him. If he's active enough to move around when you're handling him, he's strong enough to go to his water dish

Minimizing stress and keeping him undisturbed until food/med time is top priority here though. I really hope he pulls through. The prior owner should be charged with criminal neglect or something.