How often to you let him out? Every four hours? Every eight? Only when he asks? I know it's hard to let him out a lot, since you're helping your mom so much, but if it's possible, try to let him out every hour for awhile. Then every two, every three, and so on (of course, night is a different story).
Buffy is nervous, since his schedule and lifestyle have changed recently. He is obviously attached to your mom, it's clear he has adopted her as "his person." I agree that the BM are probably because of stress.
See what the vet says, but I think it's probably a matter of nerves and adjusting to a new routine. Our auxillery dog is a rescue and she was beaten before we got her. We've had her 1 1/2 hrs and it's only now that she's really coming out of her shell. She doesn't like anyone but me (I'm her favorite
), DH, and her dogwalker. That's it. If something happened to me and my mom came to stay, Sally would have a hard time adjusting to someone new in the house.