We had this problem for a short time with our second child - can't remember how old he was at the time, maybe around 2 years?
Anyway, he used to sneak in and crawl down in between me and my husband and go straight off to sleep again. To cut a long story short, he just wanted the reassurance that we were there. We made it quite clear that he could stay for a few minutes (up to 5mins) so that he didn't feel we were pushing him away, and then we very firmly took him back to bed, gave him a cuddle and a kiss goodnight and that did the trick.
We also demonstrated to him the baby listener that we still had in his room ie. we let him hear our voice through it. This meant that he knew that if he called out in the night for something we would instantly hear him and he didn't have to worry that he was cut off by himself.
This phase only lasted a couple of weeks at the most. I would just say that the most important thing is that you should be consistent - consistently take him back to his bed, even if he keeps on coming into your room and you have to get up half a dozen times to do so; once you give in and let him stay there he'll know it's possible and he'll keep trying to do it.
It's probable that he just wakes in the night, has a bit of a problem going back to sleep by himself and thinks Aha! I'll go and see Mummy! Maybe skipping an afternoon nap might help, as suggested. Maybe play a game, with him trying to help his teddy to go back to sleep? I used to sing a lullaby (badly!) to my boys and I found in the end that both my boys would sing it to themselves at night to send themselves back to sleep. Conditioning is everything!
Good luck.