Re: marijuana detection
I was the urinalysis coordinator for my command when I was in the Navy.
If your son uses it once in a great while, it'll stay in his system about 2-4 days. If he uses it often, it can stay in his system up to 3 MONTHS. (this is doing a urine test - I don't know if tested with hair, or blood.)
Hope this helps - your friendly neighborhood drug store sells home urine tests for about $25 and you'll know in about 15 minutes.