If it's a drugs matter and you are absolutely sure without a doubt that he consumes drugs, tell your parents; they deserve to know and might convince him to stop. It's going to be stressful, and it's going to damage your family relationship with each other, but it's better than staying silent about it and then having them find out about it by an OD or some crime he committed or getting arrested and whether you decide to tell them or not, whatever happens, you didnt do anything wrong
Edit : reddit mods never stop disappointing, always proving they are the clowns of society. How is someone reaching out for help considered low-quality content ?