Does it have to be one or the other?

I live with my boyfriend and his 15 yr old son. After years of a toxic relationship and/or being single, I found this man who is wonderful to me in so many ways. I am 42 and he is 46. He has a 22 yr old son as well as the 15 yr old, and he does not want to have another child. I don’t have any kids, and I have always wanted to be a mother. I get along well with his sons, but I’m not their mother or step-mother. I’m more like a cool aunt, which seems to work well for all of us.
I keep buffering about this: I feel confused and don’t know if I need to let go of the idea of having kids (maybe it’s a wish from my past, is it still something I want now, at age 42?) I feel like it’s an ultimatum – I either stay with him, or go on my own to have a child and be a mother on my own (probably with IVF if that can work). I don’t want to make that decision, and I don’t know what to do to have resolve either way – if it has to be a one or the other decision. I would so appreciate some guidance with where to start to feel resolve and decide. Thank you so much for SCS!