It will be a lot more obvious, since he's got some tankmates, when he's feeling better. Just go about your normal feeding routine with your cichlids and when he's hungry enough.. YOU'LL KNOW! lol
Oscars are really cool and personable. I want to get a couple of my own but currently I am too overstocked. I live vicariously through a friend who has a pair.
I hope you enjoy him and great work on the rescue.