As exhausting as it is, you have to bring him back to bed every time so he realizes that is what is going to happen. I didn't have major problems with this, but I have also been known to walk them back to bed and lay with them in their bed until they fall asleep and then head back to my own bed.
It sounds like he's going through a transition time right now. That may be the cause. My kids got in bed with me almost every night while my husband was serving overseas. I didn't mind it then, and amazingly, it stopped when he came home.
Hopefully after a few nights of being brought back to bed he'll stay there and you won't be tired forever!
Another thought is to offer rewards for him if he stays in bed. If it's every night, the first time he stays in his bed all night he gets something special (new toy, ice cream, etc.) Then, he gets a reward after 3 nights of staying in his bed, then after a week, then, once it's consistant, a big reward for doing it for say a month straight, and then no more rewards, it will be normal.