Not too nit pick too much but i think you mean it hurts as opposed to wont fit, if you can get a baby out the penis isnt an issue aha. As others sauid though, it just takes time, go slowly with lots of foreplay and maybed lube if you need it and youll get used to it.
Theres also ways like using a dilator that you can do at home but that probably isnt needed.
If its really bad, and his penis isnt particularly big to start with, you might look at something called vagimmosis (**** knows how its spelt) which some girls seem to have? Probably not you cant hurt to check