There is NO sure fire way to do this especially given the time frame that you specified. In fact, attempting to do this with drinks, teas, or pills -- it's just going to be a waste of time, money and worry. In my opinion, since he's the smoker and you aren't, he's got the advantage -- and the right idea. A Whizzinator and/or other methods of subbing will be the ONLY way he will pass.
I'm sorry but the fact that you think this is possible and you're attempting to provide alternatives to a method that is tried and true, well, it is almost laughable. DO NOT try and talk him out of using substitution just to appeal your "honesty" or "helpfulness" or whatever. If he's so worried about keeping his job, he NEEDS to substitute and with all due respect, he doesn't need to be asking you for advice.