E.N.
Dear A.,
If you cannot lower the mattress in the crib so that he cannot get out, keep him out of it. Do not take any chances that you will regret later.
Get him a bed asap. Get bumpers for his bed. Do not think twice about this. Do not ever think that he might not fall out, slip by accident, or whatever.
It is time to buy the gates, leave his door to his room open and gate his room.
E.