I did some really stupid things in high school (much worse than this, unless drugs are involved - I didn't touch those). Anyway, I finally got caught by my parents and was grounded and watched like a hawk, it was the best thing that could have happened to me because the more I got away with the worse I got.

On that note, I think your Mom should know but if you are able, try to help him out as much as you can so you will still have his trust, hopefully. If you could help him take care of the problem and then tell your parents afterwards would probably be the best.