You should ask your vet for a pain-killer. Metacam (meloxicam) is most commonly used in rabbits. It comes in drops and most rabbits like the taste. Aspirin can cause GI trouble and you don’t want that with a rabbit that has an upset tummy from getting the wrong kind of oral antibiotics. As you seem well aware of, pain relief is very important, a bunny in pain often stops eating and becomes lethargic, and that is very dangerous for them.
Whatever he eats now is good. Rabbits must never have empty tummies. If you can’t get him to eat solid foods, you should give him Critical Care, perhaps mized with baby food, canned pumpkin (plain, no sugar) or mashed banana. He needs his strength. If you can’t get Critical Care, you can use mashed up pellets/pellet slurry. If he won’t eat on his own, you need to syringe the food. Also make sure he keeps warm. Sick bunnys often get hypothermic, which adds to the lethargy. You can provide a hot water bottle wrapped in a blankie (but he must have enough space so he can move away from it he wants to) or sit him in your lap with a blankie on him.
I have never heard of a bunny abcess drying up and falling off of itself, but that means nothing, I’m not a vet. I’m of course a little suspicious of your vet since he prescribed the oral amoxicillin. I’ll try to look into rabbit abcesses.
Edit: I did some checking up on rabbit abcesses. Most websites like Medirabbit says an isolated abscess should be surgically removed with all of the capsule, like it were a tumor, with great care taken not to rupture the capsule. If that’s not possible it should be opened and drained and carefully debrided, then open-wound healing should ensue or the abscess will return. Long term antibiotic treatment is often needed.
This doesn’t mean your vet is wrong, I can’t say if he’s wrong because I don’t have the knowledge to do so, but if the abcess keeps growing and is causing your bunny significant pain (not all abscesses are painful to a rabbit), I’d try to find another vet. Abcesses in rabbits are different from abscesses in dogs and cats and can’t just be opened and drained, so in that, your vet was right not to open it up.
Here are some articles on rabbit abscesses that I read:
http://www.rabbitadvocates.org/newsletters/Spring_13.pdf
This one has some nasty pictures of facial abscesses, but it’s Medirabbit so I include it anyway, Medirabbit is a trusted source:
http://www.medirabbit.com/EN/Skin_diseases/Bacterial/Abscess/skin_abscess.htm
Here’s another one that speaks of surgery:
http://www.sawneeanimalclinic.com/downloads/abscesses_in_rabbits.pdf