K.R.
Stay by the bed for ten minutes, then tell him you are going to go switch the laundry and you'll be back to check on him in two minutes. Then, this is the important part, go back and check on him in two minutes! Rub his back a little bit and tell him you need to go turn on the dishwasher and you'll be back in five minutes, and do it. By 3 1/2 he probably won't freak out and start crying when you leave, especially since he knows you are coming back. Once you have set this as a pattern, he will know every night that when you say you will come back, you WILL. This will keep him in his bed because he doesn't feel the need to come find you, or make sure you know he's waiting for you. We started this with our son around the same age, and I also had a newborn. Eventually he will fall asleep waiting for you. At six years old I still tell my son I will come back and check on him, and I do one check at the five minute mark and then after that I am done for the night (I also tell him I will come give him a kiss after he is asleep, and I do:)). Now if only I could get my two year old to sleep on her own.... haha!