No you're not over worrying. Time should be spent preparing for College. There's a big jump between the amount of independent study required. A lot of responsibility is with the student.
If he's not listening to you, what about asking either Dad, an elder sibling/cousin/friend whose been through that transition or another relative. Especially someone his age as they can highlight the problems he'll face when in college. Problems that will force him to grow.
Try to get him establish a regular sleeping routine for him. If he doesn't do any exercise, try to get him out of the house moving about as that will help with sleep. Go for a jog with friends as an example. With regards to the gaming, emphasis to him balance. Its ok to play games when he's done x,y,z not the other way round. Games now can be very addictive so it he may find it hard to immediately quit. Slowly reduce the gaming time. From 4hrs to 2 for example. Or split the sessions.