I'm not a Doctor or a HR type, but if you are on prescribed medication and declare it before you take the test, then it would only be an issue if the effects of that medication would make it unsafe to perform your duties. I once took a routine drugs test while on Clarithromycin (an antibiotic) and I failed for Cocaine (!) but because I'd declared it I was fine.
I wouldn't let him put it off, but when he does apply, I'd make sure he does declare it when he's asked because finding it in the test results otherwise might make it look a bit dodgy.