I need advise with my 21 year old son.?

My son is 21 yrs. old. He stays in his room all the time and is always on his computer, has no friends and is steadily gaining weight. He has quit several jobs, dropped out of college and seems to not care about anything. I know this is some form of depression, I need some suggestions in getting him some help. He is very smart and never been in any trouble. Does not smoke or drink. Any advise or suggestions will be greatly appreciated.