there's still a ton of things that can cause that.
Symptom Checker
here's a symptom checker you might be interested in. there's tons of possibilities. of course, this is in no way intended to be a substitute for his dr's visit. this is not meant to actually diagnose anyone. only to help you narrow down possibilities before seeing a dr. so you don't have to worry that it's something worse than it is.
you still haven't provided much details. you haven't given us any indication of how long this has been going on. or how much weight "a lot" is. or if he's experiencing any other symptoms. it could even be something like a flu with as little detail as we have to go on.