The first thing a sleep expert would want to know is if your child is experiencing any behavioral issues at bedtime. If getting him to fall asleep is often a struggle and then you give in and lie down with him, your child probably just needs some sleep training exercises to learn how to doze off on his own. Start by remaining in the room with your son after his bedtime ritual while he falls asleep -- but not in his bed. Be sure he doesn't see this as a punishment, but rather part of becoming a big boy. Sit in a chair or in a corner and be completely silent (this should be a boring time for your son, which means no talking or eye contact). After about a week of this, start leaving the room when he's very drowsy, whispering goodnight as you go so he doesn't wake up expecting to see you. If he calls you back or gets out of bed, go to him or walk him back to his room, keeping the interaction uninteresting -- remember not to speak, just adjust the covers and leave again.
It might also help to create a bedtime chart and give your son a sticker for every night he falls asleep (and stays asleep) on his own. After five stickers or so, reward him with something small, like going for a bike ride or a trip to the library. You might also want to consider making your son's bedtime a little earlier, since kids who are overtired usually have a much harder time dozing off and don't sleep as soundly.