Ouch. As a living-with-mom guy myself, who doesn't like - and has recently relearned that - social events, I'd suggest just letting him know how you feel. Don't tell him that he can't have a good time, of course, just let him know you wish you were as well. Nothing may change, but talking through it can sometimes help with the direct issue, which is feeling lonely and "different" for being depressed and anti-social.

*hugs*