It could be separation anxiety. They go through this several times at different age points as they really put together who they are in world and others' place in their world. Not what you want to hear, probably, but it really could be a true need. But, hopefully he will work through this developmental burst and get back on track soon. :)
We went through that at 18 months (as did many of the other 18 month olds on the birth club I was on). I have come to accept that good or bad sleep is all a phase.