Going down on him (oral)

So guys this is kinda personal. I am the worst at sucking penis. I try to go down there on my SO but I gag. It's not because he's stinks, it doesn't gross me out. It's just my reflexes. I gag when I brush my teeth!! Any advice on how to go down there without gagging as much? Does anyone else have this problem? By the way, he doesn't force me to do it, he doesn't even ask me to do it. He asks sometimes and I don't mind doing it but sometimes I wish I could please him just by giving him oral, especially when AF is around for a visit. Please any advice would help!!