It's a very very very hard thing to do....but LET him cry.

Encourage him before bed...say "It's time to sleep like a big boy, all by yourself". Prepare him an hour before bed, say it over and over. Let him know it's time. He might be young, but I'm sure he understands night night time or crib.

It's okay to let him cry, it's not as though he needs anything or is hurt. It just seems that way.

My son started doing that "Can't sleep unless Mom's laying next to me" stuff, I had to nip that in the rear though because I do my homework after they go to bed. Basically I warned him, made my tuck ins shorter and shorter, then let him cry. Eventually his crying went from forever, to shorter, to no more.

Good luck, hope everything works out.