How do you get your boyfriend to put in your tampon?

It depends on whether or not you want to let him do that for you.

Your body is your own, thus you decide who touches you and how - or if they touch you at all. Some people wouldn't have a problem with this and to some people this is intimate or arousing, but that doesn't mean that you should agree to it and you shouldn't agree to it just to keep him happy either.