I want to sing on my friends track
Sounds cool, why do you want to contribute?
Because he promised I'd be the first one to be in it when he releases his music
While its good that you want to be on the same track as your friend, you may just want to allow your friend to  have the spotlight in this scenario. As long as your friend is adding you to the song to help the song instead of to help you, you are fine. Putting your friend first here  however, may be your best decision as the end of the day.
I don't know if he'd help me with my own.
It's quite exciting to be on a vocal track as a guest, but I believe it pays to be a little more modest about it. Overshadowing him can possibly ruin the track, and he may be disappointed about it. How about just supporting him on the sidelines, he would really love it and your friendship would get even better.