I have been in your situation before and to tell you the truth, I think it's best that you don't tell them
#1.Blood is thicker then water and his parents may not even believe you. Heck, it took my inlaws about a month before they would believe me and they knew of my AH's past drug problems.. Even if they did believe you, there is not a whole lot they can do about it... No one can force anyone else to get help.
#2. He will tell on his ownself sooner then later.... One can only hide drug abuse for so long... eventually it starts showing itself..
The only way I give out information now is if my mother in law asks me questions... I do tell her the straight up truth about what is going on because one of my boundaries is not to lie for my AH anymore. But I only tell when she asks..
Keep the focus on you and getting yourself the help and self nurture that you need...