Just sit him down and tell him you're concerned about him and mention why- what behaviours of his have changed lately. Asking him how he's feeling because if he's down you'd like to help.

Enlist his family and friends for support. It's good to do something before it gets severe. Encourage him to go to his doctor.

It can only be classified as depression if it's been going on a long time in which case reading books on depression or consulting a therapist might help. But if it's only been a few weeks he might just be going through a rough patch or be upset by something. If you can get him to communicate - through gentle persuasion without pressure- you'll have all the facts& you'll be able to help, so you won't worry as much. But ultimately, he has to help himself.