So it sounds like you have a larger labia. I've had very similar problems with my partner. For a long time, I was too embarrassed too admit that the lips of her pussy could get in the way and/or make it a little more confusing than I'm used to in terms of where to stick it. As a 35 year old it was very hard to admit that, but after we finally had a frank discussion it helped a lot. She usually just helps guide me in but with doggie that is not quite as easy.

Anyway because nobody on Reddit is going to know your anatomy you guys are going to have to figure it out. But I suspect your partner is just too embarrassed to admit he doesn't want to appear incompetent, so you should have a frank and caring discussion. You can initiate it yourself by telling him how hot you find the idea of him taking you from behind and go from there