Sounds like he’s hitting your cervix, which is the thing that marks the end of the vagina and the start of the uterus.
Two suggestions for you:
1. Tell him not to thrust as deeply. You can try positions that require shallower thrusts, too, like spooning or “lazy dog” (you lie flat on your stomach with a pillow under your hips and he enters you from behind).
2. Engage in a lot more foreplay before sex. The cervix moves up and back when you’re aroused, so sex will be more comfortable if you get thoroughly turned on before any penetration happens. You could even go as far as having an orgasm or two before penetration starts.
I hope that helps, and I hope your boyfriend respects your needs in order to avoid hurting you in future!