I agree, Definatley seporate him! you can try getting earth worms and cutting them (ewww...) up into smaller pieces, it seems to work well to get them eating again.
Dont worry too much about getting him to eat ASAP, he will eat when he feels
1)less stressed, very secure (his tankmate might be stressing him out, or the tank mate is getting all the "easy" food first)
2)healthy and is parasite free (so maybe get him to the vet for a little check up)
Keep us posted, and try to post some pics if you can, it always helps us diagnose how bad/far along your frogs problem is, so we can reccomend a more/less drastic approach!
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