you could maybe stick him in a QT? by himself, then you can keep a better eye on him, see whether he is eating or not, and to jumpstart the process, try giving him a few live foods to make him abit more active, go to the LFS and buy like 5 lil cheap fish that he can happily munch on.
like some goldfish or something?
it'll go crazy for a little goldfish.
put about 10-20 in a bowl, dont have to look after them really, and then give the lion 1 a day or every 2 days, then when he looks better you could re introduce him into the tank?
worth a try?