When I transitioned my daughter into a big kid bed, I lowered the drop-down side of her crib and had a twin bed butted up against that side of the crib. So, if she did climb out, she was in the big girl bed (with a safety railing of course). We talked up the big girl bed and how she was getting to be such a big girl! Some nights she opted for the bed, some for the crib and sometimes we found her in a different place in the morning than we'd put her the previous night. Then, I just told her that the crib was going bye-bye and one day it did. You have a prefect situation with passing it on to the baby! You can make your son feel so great and involved by talking up the fact that he's giving the new baby a BED! What a nice thing to do!!! What a big boy!!! BTW, baby-gating the bedroom door will keep him in his room at night.
Congratulations on the new one!
D.