Dear Amanda,
What I am about to say mat not be very satisfying. There are a few youngsters who display neurologically-based sleep problems, but such children are few and far between. But, to rule out that there is anything medically going on, you should (if you haven't already) have your son seen by his pediatrician.
It is more than likely that he is healthy, and that his sleep pattern is disturbed because he has learned some habits which have become a problem. In part, I say this because you mentioned that you bring him into your bed. This is one sign that you may be contributing to the problem, even though, of course, you don't intentionally do this.
Have you read Richard Ferber's Solve Your Child's Sleep Problems? If not, please do. It is a treasure for parents who are faced with the situation you are facing. If you follow the prescribed regimen, your son will sleep according to a more
normalNormal saline flush pattern.