As others indicate, legal substances can be the basis for DUI. Oh, like alcohol.
There is no driving pattern if he was stopped for an inspection sticker, but beware that the police report might be "reinforced" with additional observations.
The police audio/video would be very important here because it is a memorialization of what actually happened. A jury gets to decide on their own if the person sounds or looks impaired.
In my state, and likely most others, the state has to prove more than just the presence of the substance and driving a car. The state has to show impairment caused by the substance. Otherwise, someone could get a DUI based on taking an aspirin.
You likely want third-party testing, and you'd want testing that shows the amount of the substance in the body, if possible. A test that is just positive/negative (qualitative) is not helpful. A test that measures amounts (quantitative) can show an amount that allows a doc, or possibly a chemist, to testify that the amount is insufficient to cause intoxication.
Best way to find an attorney is word of mouth. Or local news articles about attorneys who actually went to trial, regardless of whether they win or not. (The best attorney in the world cannot beat an unbeatable case.)
And be prepared for the possibility that your son is not being entirely truthful. The pot DUI was maybe a surprise, I'm guessing.