You are not going to keep him out of that crib no matter what you do. Rie had the same problem when they put up a bed for Dakota.. It is "new" and once the newness wears off, he may just move on to greener places to sleep so let him get bored with it and lay in there until almost time for the baby to come.... THEN offer him something new to lay on..

I had kitties with all my babies. When my oldest son, I had a cat that use to get in bed with him when he cried. The kitty would stay there until he went to sleep, then would get out and go on to other things. It was a babysitting kitty and did a wonderful job.
Cats - babies... WHAT a wonderful world!