Staying in his room, sleeping too much, and losing interest in things he used to enjoy are pretty common signs that something's off. But there's more to watch for: changes in appetite, dropping grades at school, constant irritability, talking about death or hurting himself, neglecting personal hygiene, or spending way too much time on his phone/internet. Adolescence is a pretty wild phase, but when several signs show up together and last for weeks, that's when it's worth getting help.
We know it's hard to tell whether it's just "annoying teenage phase" or actual depression, anxiety, or something more serious. If he's been like this for more than two weeks and you're seeing a real change in his behavior, there's no harm in talking to a psychologist or taking him to the pediatrician for an evaluation. It's not a waste of time, and usually you can pretty quickly tell if he needs ongoing support or if it's just a rough patch.
The important thing is trying to talk to him without sounding accusatory, you know? Sometimes teenagers shut the door precisely because they feel their parents are pressuring them or judging them. If you can create an atmosphere where he feels safe to talk, then you get a better sense of what's going on. But if he won't open up or you keep worrying, just go straight to a professional - don't put it off.