A. With all due respect, son may have been smoking himself. B. Dad smokes regularly.
Stop asking for samples he won't give you. Talk to your lawyer and figure out how to legally proceed. At any time Dad will fail a drug test, hair or pee (it's very easy to pass unsupervised pee tests, so if he does a pee test somebody has to watch him pee). Since his passing a test is a non-issue, you just have to proceed in a healthy, safe, and legal way.
Don't demand hair or pee out of anger or to "prove" you are right, and don't expect to get it. That's not rational. He's a smoker, you know it, and you know what pot smells like. He may be stoned, but he's not going to willingly hang himself.
My two cents.
Cyranoak