It sounds like something is surely hitting your cervix. I suppose you might not notice it while you are doing it because your brain releases endorphins to kill the pain. However my wife used to definitely feel it if I hit her cervix when inside her and it HURT. I could play with it with my finger and she liked that but if I bumped it with my penis she did not like that at all. So either his penis or the dildo is certainly hitting it.
Why does he want to put both his penis and a dildo inside you? Does it turn him on to think about stretching you? I never thought of doing anything like that. I didn't want my penis rubbing against a dildo. I wanted pure vagina grasping it.
Your vagina will grasp whatever is put into it and it doesn't have to be large for it to grip it tightly. I suppose there are some girls who have stretched theirs or have weakened muscles so they need something larger for it to fit tight but most do not.
I think your cervix gets larger during sex to try to capture any sperm that might be looking for a home.
I think he may be using the dildo because he feels you need it inside you to be satisfied but I'm no psychological analyst.
I never saw penetration as the most important part of sex. For me, what most people call foreplay was the big attraction. I worked on my wifes vulva with my fingers and tongue and played inside her with my fingers. It pleased me so much to see her orgasm and know that I did that, that I would orgasm at the same time even though no one was touching my penis. She liked what I was doing and after the orgasm I would put it inside her ahd we would just hold close and enjoy being close. Sometimes we would go for a second orgasm with me inside her which was also wonderful of course.
For us it always worked better to satisfy her manually (tongue u aly?) and then do insertion for the feeling of closeness.
I think you just need to work out what you and he can do to please both of you. Right now it seems as though he has the idea that he has to put something big inside you to make you happy so that's what he's doing. If you can be pleased with outside stimulation and he can do it, I think you will solve the problem.
It's not like my wife and I jumped into bed on our wedding night and everything was perfect. It took years to achieve what we felt was the best for us. Not 20 years but more than a year. But even after 20 years we kept improving on it, or adjusting to what we liked at that time.
If you can find a position where you can grasp his penis in your vagina just for the feeling of doing that so you can both enjoy that, I think that should help a lot and you can do the orgasms manually. I feel doggy style and on her back with her knees up by her shoulders were the positions that shortened her vagina the most. I can't tell you if either gave me better access to the opening though. I'm sure with a little exploration you can figure out what works best for you.