Feeling sad/jealous of partner’s past life

Looking for some perspective on this. I’ve been with my partner for about a year now. He’s got a 13 year old son who I get along with really well and I really enjoy spending time with both of them or even just hanging out with the kid. He’s really cool and I love listening to him talk about his interests.

I never wanted to have kids and my partner got snipped when we first started dating which I really appreciate. I’ve never dated someone with a kid before but I figured that now that I’m in my mid-30’s, most men my age would probably have kids and I could make it work if the kid was older.

I’m having some feelings come up that I never anticipated. I’m kind of hurt/sad/jealous that he has shared a bond or experience with someone that we will never have. I don’t know why this is bothering me so much.

I’ve tried to talk to him about it a few times but he always says he doesn’t want another kid. I don’t want a kid either! There is nothing appealing to me about pregnancy or raising a kid or taking care of a baby. It’s been making me feel really misunderstood and disconnected from him emotionally. I just want him to understand how I feel I guess but he just keeps interpreting it as I want a kid? I’ve been trying to just let it go but this keeps coming up occasionally and I don’t want to try to discuss it with him anymore because he doesn’t get it and I don’t want to upset him and ruin the day/evening with him about it.

Has this come up for anyone else? How did you deal with it? Am I being totally out of line for feeling like this?