Just tell him. It's really no big deal when you think about it. It's a cross between working out (getting bigger) and jacking off, and a lot of guys do both of these things. It's about being an authentic you. It's what you do, and who gives a shit what other people think. You know it works. If he wants to try it for himself, fine. That's not the purpose of telling him though. The purpose is so you don't have sneak around your own place and worry about getting busted doing Penis Enlargement. If he's a cool friend, it won't bother him, and you should be able to trust him. If he tells anyone, just say so what, all guys jack off, this is not that different. It all is in how you explain it.