I recommend you get your son to read this, op, or at least bits of it.....
linky
It's the actual review that all the papers have been making a fuss over.
"In summary, the epidemiological and laboratory evidence on the acute effects of cannabis suggests strongly that cannabis users who drive while intoxicated increase their risk of motor vehicle crashes 2–3 times as against 6–15 times for comparable intoxicating doses of alcohol. Cannabis use was estimated to account for 2.5% of traffic deaths in France as against 29% for alcohol. The risk of an accident increases substantially if cannabis users also have elevated blood alcohol levels."
Also:
"First, the decline in IQ was largest in those who began using cannabis in adolescence and continued near-daily use throughout adulthood.
......
Fourthly, there was some recovery if users quit using for a year or more. There was no IQ decline in cannabis users who started in young adulthood and had not used for a year or more before follow-up.
........
It is worth stressing two things about this study. First, these effects on IQ were found only in the small proportion of cannabis users who initiated in adolescence and persisted in daily use throughout their 20s and into their 30s.
Secondly, the 8-point decline in IQ in the heavy sustained users was not trivial: it was half a standard deviation lower than their peers. This means that the average IQ of these heavy users was below 70% of their peer group."
I wouldn't want to let my son drive either, if I were you.
I don't have any practical suggestions but I do worry about this stuff in the future with my own DC (currently toddlers) :(