#1 - relax!
#2 - Don't anticipate problems. Your son will pick up on your anxiety
#3 - Go with the flow. If he does go in your room or get upset will that change the course of events? No, you'll still have your new baby.
#4 - Remember you probably didn't get a lot of sleep with the first one so you may be up anyway with baby #2.
Start off with telling your son he's getting so big and getting him to buy into the big boy bed idea.
I never had a baby monitor (I'm a super light sleeper) but if you use one, close your door at night so he has to knock to get your attention and it's not o.k. to just waltz in, whenever.
Try to make special time for just your son, when putting him to bed. I found that spending some special time (and I tell him 5 or 2 minutes or however long you want to give) saves 3 trips up and down the stairs.
Each child is different. With my daughter, even when she was an only, I kissed her goodnite and walked out the door. My son (now 10) likes his nightlight on, music or "white noise" playing, back rubbed, etc. We talk then and SO much comes out.
You're lucky they are close in age. Enjoy your precious babies -- they grow up so fast!