Good info, thanks.Not gonna get into a pissing contest over this particular stop, but I will tell you about vehicle stops and MJ odors in general.
In general, the odor of MJ alone may not establish probable cause for a search of a car. However, if a trained K9 "alerts" on the car in the handlers judgment, that and the smell the officer detected will likely be enough to establish PC.
And keep in mind, a dog can alert on odors a human will never sense. So, if someone was riding with your son in his car a week ago, and had the odor of marijuana on his person, that might still be detected by the dog. And possibly by an officer depending on circumstances. It's possible. It could also be possible a seed or some shake from the weed might have fallen off that person on to the floor, seats, etc. One would likely not even notice it or recognize it but a dog could easily smell it. That's also entirely possible...I know because I've seen it on numerous occasions.
Once that PC is established a warrantless search may be conducted into any place within the vehicle where the contraband might be concealed (in this case, marijuana). So the officer could pretty much search the person and vehicle along with contents of the vehicle. They wouldn't be allowed, for example, to look thru his phone...because MJ can't be hidden in a phone.