Positive Discipline

Dear Dr. Markel,
I have a 3 and a half year old little boy and a 6month old little boy. The 3.5 year old was breastfed until he was 2.5 and slept in our bed until he was 2. Since then, he sleeps in his own bed until somewhere between 2 am and 6am and then stumbles into our bed. Until we had the new baby, this was fine. now however, I would like him to stay in his own bed and not wake up in the night. For the past month he has been waking up several times a night and coming into our room. We either walk him back into his room and wait for him to go back to sleep or my husband or i get in bed with him and go to sleep, or he sleeps in our bed. For me, as i am already awake many times in the night to feed the baby, this is too much. We have tried rewards, threats and all sorts of thing to try and let us all get more sleep. What would you suggest to help the situation? Why do you think he's getting up so often? Thanks for your help!