The same way you would reject him if he asked you to start smoking. Just say it's not something you are looking for, and that you are satisfied without it. But thank him for taking time out to show it to you, because unlike smoking, he is only recommending these because he wants to help. After which, go for some event together/do something, so that the memory of whatever you guys do together will over run the memory of "rejection". Thus making a much less awkward follow-up encounter. If he is insisting on it, then you have to think through why he is pushing something that you feel is not good for you, on you. If he wants to keep the friendship, I don't think he will do that though!