I think my partner wants more kids but I don’t. What do I do?

I’m 33 and I have a nearly 15 year old a 9 year old (both from a previous relationship) and then a 5 month old with my current partner. I was always adamant after my second I didn’t want anymore but that changed when I got with my current partner (he don’t have any kids and I come around to the idea) now he knows I am done and don’t want anymore but yesterday he suggested we should have another. When I asked him if he meant that he joked that he didn’t and was just joking but deep down I don’t think he was. How do I approach this and get him to say honestly how he feels he said he is happy with the family he has and is happy not to extend it but I don’t want him resenting me in a few years because he hasn’t been honest now. I don’t know how to handle this situation. Any advice is welcomed. Thank you.