I would, without being accusatory, see if you can find out info about what the program is going to be about. My guess is that it's focusing much less on the actual drugs themselves and is more about making good choices, staying safe, staying healthy, and what to do if someone at home is using drugs and alcohol. If that's the case, I would just reinforce the choices that he's probably already making - the right ways to treat our bodies, what to do when someone isn't safe, etc.
While six is young, there's really no way for the school to win here. Either they're too young because they haven't been exposed to drugs and alcohol yet, or they're too old because they have had the exposure and haven't been educated. And while in an ideal world all kids would be getting a consistent "drugs and alcohol are bad" message from their parents, we all know that simply doesn't happen for a variety of reasons. But I 100% promise you that learning about drugs and alcohol won't make him more likely to try it, especially at age 6! Honestly, it might not make him _less_ likely to try them as he gets older either, but, in my opinion, it's better that we arm our kids with the developmentally-appropriate facts than to simply hope for the best.
Good luck.