It's hard to say for sure why he isn't eating. It could be a breeding season thing, it could be the fact that he just moved to a new home, it could be that he's stressed about the tank (if he's used to a rack/tub)....

As for the new addition, DO NOT put them together without quarantine. 3 mos as a minimum, but we use 6 mos, and some people do a year.