here's the problem with drug testing another adult...
1) they can refuse which does not necessarily imply guilt
2) the test may give a false negative or a false positive, they are not fool proof
3) if it IS positive, he can deny deny deny which may or may not be true
4) if it is negative that does not mean he did not use but perhaps found a way to trick the test
in NONE of the above do you get an absolute verification of the TRUTH.
have you considered just sharing your concerns/doubts WITH him?