K.B.
Have you looked into a crib tent. We got one for our daughter a few months ago. We have the one that goes on top and covers the sides. My daughter is now 12 1/2 months old and can stand up in it.
With the sides, you can reach the side bar of the crib, which may prevent him from standing up until he can get up and down with ease.
I got mine used on Craigslist. It was much cheaper than brand new....