hi Lisa,
I recognize a lot of myself in your son. When my depression was at its worst, I was sleeping up to 20 hours a day, I hardly ate and took next to no pleasure in food, I was distant and cried a lot ... I just want you to know that there is hope for your son -- depression is extremely treatable.
I think you're doing the right thing by being there for him. I wish my parents had been as observant as you are and picked up on my symptoms -- or if they did, they never made any sign of it. Continue being supportive of him, that's the absolute best thing for him now. When we're depressed, our thoughts tend to warp to the negative -- we believe we're alone, we tend to hate ourselves or blame ourselves for everything, we lose interest in a lot of the things that used to give us pleasure. You're doing the right thing by just being there for your son. Make sure he knows that you love him and support him, and keep telling him that because the depression may make it difficult for him to believe. You're doing well not to nag him -- my parents would nag me whenever I got into a slump (I was being "lazy/inconsiderate/impossible/anti-social," etc) and it just made things worse. Give him a bit of breathing space, gentle encouragement to get out of bed or go for a walk or talk about things with you is good, just don't pester him. I also wondered if you've talked to him about this? Maybe you could let him know that you're concerned, and you think he might be depressed, and if he wants you and he could explore possible treatment options to make things better for him. Again, don't pressure him, just let him know that you're there and willing to do whatever he needs you to in order to help.
The last thing is that the internet has been hugely helpful to me, and it's good that he has that at his disposal as well. Don't take it away from him, no matter what! Besides internet friends, who are a great way of talking out his feelings, the internet is also a good distraction from how he's feeling. I use the internet to get out of my own head a lot of the time by playing games, reading articles, contacting friends... it's a great diversion, all in all.
Good luck, and please let us know if you need anything else. PC has great ressources on depression, I'm glad you've come here.