Doggy style only

So me and my boyfriend have only been having sex for like a month now and we’ve only been doing doggy style (which is fine) because that’s the only position that he will actually fit in. I really want to try more positions but any other positions we try it just won’t go in. Why is this happening and how do I fix it? Also we don’t use condoms btw, don’t know if that changes anything 🤷🏽‍♀️