If he suffers from insomnia, I guess it has more to do with tiredness than motivation.
I don't know if that's your son's case (or yours), but playing video games or watching TV before going to bed is a big factor in insomnia as it changes melatonin levels. If he does that, I think it's the first thing you should try to change
As someone else suggested, reading before going to bed is a good thing.
As for eating before going to bed, there are different theories. (I have a childhood friend who is a nutritionist.) Personally, I need to eat less than two hours before bed, or else I'll wake up in the middle of the night because I'm too hungry. So it depends on people.
The number of required sleeping hours are also different from one person to another. I know people who sleep only 5 hours per day, while some need much more. Einstein, for example, used to sleep 11 hours a day. I know it might be difficult for a teenager to go to bed at 8 or 9 pm, but it may be possible that it's exactly what he needs.
Hope that helps.
Edit: As you have probably guessed, the problem with video games and TV come from the screen and not from what's on the screen. So chatting or reading Gamebanshee threads before bed is not recommanded either.
[font="Book Antiqua"][color="DarkOrange"]Dr. Stein grows funny creatures, lets them run into the night.
They become GameBanshee members, and their time is right.
- inspired by an Helloween song[/color][/font]