I (thankfully) haven't had any experience with this - so unfortunately I am NOT the best person to advise you!

It may be wise to get him checked by the vet again - just because he was OK last time doesn't guarantee that this time the reason is the same.

Did you do anything to "make" him stop last time, or did he just snap out of it on his own?

Did he do it in the same place last time?

I KNOW it would be a hassle, but is there any chance you could put a temporary screen door up on your mom's room - this would keep him out and still allow air to circulate.

Sorry I can't be more help! Good luck!