[Usernames Suck]

There's a number of reasons why not but what matters most is this: he doesn't feel comfortable doing it. Therefore, he shouldn't be pressured into it. Sometimes certain activities just aren't appealing to people, maybe there's a reason, maybe they just aren't, but they have every right to not do something they don't want to do. As others have said, this works both ways, and if there's something you don't feel completely comfortable doing, you also have the right to refuse to do it and he shouldn't pressure you either.

The most you can do is talk to him openly and ask him about it. Don't pressure him and don't judge him, creating an environment where both of you can talk without being afraid of being judged or pressured is important. If you have this, he may feel safe enough to tell you why not if there's another reason and he may feel comfortable enough to find a way to work with you on it, like if he just doesn't feel confident and thinks he won't do well.

However, it's completely possible that he just doesn't find that appealing or doesn't feel comfortable doing it or talking about yet and that's fine too. Just as it's fine if there's something you don't want to do for any reason at all.