If he comes back and his eye isnt healed yet, keep him inside until his eye heals. That way you know his eye is ok.

Hopefully he's just being held captive at your neighbor's house and hasnt had a bad run in with a moose or some other cats.

Did you ask your neighbor if she's seen him. You could tell her you're worried about the cat because he was on medication for the eye infectioin and you want to find him so that you can finish the treatment on his eye.