WELCOME mmlj!! I'm sorry to hear about your bunny. The crusty eyes could mean a number of things, and should be seen by a vet. It could be a simple infection to something more serious. But either way, the type of illness should be diagnosed, so that the correct antibiotics are given.
But the fact that he is lethargic means you need to take your bunny to a vet ASAP. Is your bunny eating and pooping normally?
Lethargy can many times mean a bunny is getting into or is already in a serious state.
Please keep us updated! I am sending healthy happy vibes you and your bunny's way.