Maybe double up on bumpers? Not width wise but length wise?
This is the only thing I can think of. If you put a second bumper on top of the first, then he'd REALLY have to try to get his legs through the slits.
I think my parents got a crib tent for my youngest brother. It was set up inside the crib, but had the mattress 'at the bottom' to anchor it down. It looked super weird but I don't think he could stick his legs through anymore. *disclaimer* this was about 7 or 8 years ago that my parents did this. I don't know if it would still be considered safe by today's standards.