If you still have problems, you could always use the technique one woman resorted to. She ended up have to strap her three sons into bed.
Of course this was many, many years ago. It's really funny if you think about it, although, they were much, much older than your little one.
I know a seven-year-old who can't stay in bed at night. Some things work for some children and don't for others.
Prizes will definitely work in most cases, and always making sure they have their favorite toy. The only problem with sitting with them is that if they don't fall asleep, you're stuck there for the rest of naptime. If you were going to sit with him, you could bring your knitting in with you, just in case.