Can i get a follicle hair drug test on my 21yr old son, without him knowing?

Probably: Since test kits are available on line, i suspect you could get the testing done. All you need is an appropriate sample. But why not just confront him with your concerns and ask for a sample. Doing it behind his back will only lead to resentment. I can see nothing to be gained by the plan that you are contemplating except hard feelings and his denial of whatever results you obtain.