I work in a medium secure unit and we test the patients for drugs from time to time. My understanding is THC (cannabis) can be detected up to 30 days after use, drugs like heroin are clear much sooner.
According to this
https://www.facebook.com/TheLCC/posts/10154005608035014
"The reality is that after just a couple of hours, somebody who has consumed cannabis would most likely have less than the legal limit of 'THC' in their system."
No disrespect to the OP ( and you know your son whilst I don't)but I have heard it all when it comes to drugs "my drink was spiked" "i am holding it for a mate" "i tried it once and its not for me", if you are hanging around with users chances are you are using too.