Hi C.,
You have learned what a lot of 2nd time mom's have (me included). Taking the "shortcut" to get them to sleep only works for a while, but then it can't be sustained.
One question I have you did not address in your post: What did you do at bedtime before he was moved out of the crib? Did he go into the crib still awake? Or did you rock him to sleep? What was the routine?
I learned (a little late, but in time for #2) that it is best to train them from very early on (by 6 months) to go into the crib still awake. Drowsy, but not yet asleep. This lets them learn how to go to sleep without you right there. This is a very important skill (as you have found out). There are a lot of different books and methods about how to do this (I did Ferber with our daughter), but the important thing (after ensuring that they are awake when they go into the bed) is that you are consistent in whatever method you choose.
With my first child (son, now 10 yrs old), he was an early walker/climber/crib jumper.. and was on the crib mattress on the floor by about 15 months old. He didn't have the self control to stay in his room on the bed to go to sleep. I had (thought I had) to lie down with him each night. This sometimes dragged into hours long when I was exhausted. But, I was exhausted and lying down was "easier" than the alternative... for a while. Eventually (baby #2) things had to change.
You have gotten good advice about slowly moving out of the room, not talking after he gets up, etc. But I don't think anyone has really stressed the point about him being awake when you leave the room. With my son, I planned a 5 minute routine for when the light went out. After the whole prayers, story, last sip of water stuff, the lights were turned out. I layed down on the bed with him and sang him the lullabye I had sung since his infancy - a minute or so long. Then he liked to snuggle with an arm around my neck. At that point, I told him that in 3 minutes I was getting up to go ___ (fold laundry, go through the mail, clean up the kitchen, anything that he could HEAR me doing so he could picture me doing it). Then I counted in my head to 180, said, "it's time for me to __" , gave him one last kiss, "retucked" him and left. Occasionally he would call to me, but I never came back past the doorway. It became so easy.
At first, you will have to be strong and consistent, walking him back when he pops out of bed, just like the other moms said. But if you are consistent, he will learn the routine and just go to sleep.
Don't feel bad about having fallen into the routine you have now, lots of us have done the same things... Just remember when the new baby comes what the longer term consequences can be, when we indulge in what seems "easy" right now. :))
By the way, my husband also was often already gone to bed when "bedtime" rolled around, so I know the exasperation and exhaustion you are experiencing! Day in, day out. Every night. You and your son. So many times it is helpful to have Dad provide an alternate way of doing things, but if they are not available, there is not much help they can be.
And YES, you do have time to work this out before your new baby makes its arrival! Don't stress about it.. no sense crying over spilled milk, as they say. Just go from here forward and create a new routine.
God Bless you. It is tough.
Stay strong!