If he is writing most of the songs, then that can make it awkward but it can also be an opportunity.

If you are willing to sing his songs and be the frontman, he may end up loving it, if he can be more of a backing vocals/guitarist. Most people take a lot of pride in the songs that they write, and mostly they just want to share them and for people to enjoy listening to them. So if it was ME, and I wrote some good songs, I'd want them to be performed as well as possible, even if that meant someone else singing. Of course, he has to agree that you're singing it better than him, but that would be where video of shows/practices, and/or votes from other band members. Good luck