My daughter used to get out of bed over and over. What finally worked was the first time she would get out, I would tell her it was her warning and that if it happened again, she would have to sit in time-out. I would take her back to bed and, of course, a few minutes later she would be back. I would sit her in time out (and make sure its in a room that you are not in and that it is pretty boring, for us, we use our dining room). Then I after a few minutes, I would tell her that she could get out when she is ready to go to bed and stay in there and go to sleep. Sometimes she would sit there a while longer but eventually she would tell me she wanted to go to bed and she would finally stay there. This went on for probably about a week and then she stopped getting out because I think she finally realized it was either bed or time-out and I guess her bed seemed to be the best of the two.
I think I would just tell him when he tries it that you love him very much and that's why you want to make sure he gets plenty of sleep so he can grow up big and strong and then give him options - stay in bed or sit in time out (and hopefully he will eventually pick bed too.) Hang in there and good luck!