My MIL was living in an assisted living facility when my stepson would "visit" her to "say hello" and spend time with his grannie--aka...mooching food, money, shower from her. *sigh*
Legally, the facility could not block him from visiting, but we did arrange to receive a phone call if he ever showed up. That cut those visits off pretty quickly.
I am not sure how you will be able to convince your Mom to not enable your son. However, if he becomes dangerous to her, it is possible to get your local elder care services involved.
Sorry about all the drama...hang in there!