If you're talking about while he's masturbating that wouldn't help because then he wouldn't be able to hear how loud he's being. The best option would be to turn music on out loud to hide the sound from his mom.

Also, if you're talking about getting away from the family and muffling the sound of his mom talking about him, I don't think that would be very comforting. Then he doesn't hear what she's saying and gets to wonder about it later instead of standing up for himself.

Or spank the ferret.

Or pet the duck. Edit: Oops, sorry. I thought we were talking about actual animals here.

We weren't?!

If you pet the duck too roughly, you'll quack it.

Or bash the bishop.

You could always "wax the carrot"

Maybe Mommy catching him is what puts the pop into his masturbating.

With that name, I wonder what you might be doing too much of.