I wouldn't worry about your son. He's little more than a toddler and children, throughout childhood - and often in adulthood - go through stages of wishing they had more or less, or no, siblings. It was unfair of your dh to mention it to him really, but as he is so little he will very easily be distracted if allowed to forget it. However, many people do mention siblings to children a fair amount - even strangers. And they do easily move on from the fixation ime.
It's your own disappointment that is driving your feelings really and that's much harder to deal with. You absolutely cannot push your dh into having a second child, if he doesn't want one. That could potentially lead to issues later down the line.
You do have choices.
If it's really important to you, you can move on from your husband and seek another relationship where a baby will be part of it. You wouldn't be the first, or last, to make that difficult choice.
Or you can try to come to terms with the disappointment. It will be a form of 'grief' in some ways to change your thoughts of your further. But you can work through them and make new dreams for your family's future.
I have an only child. She is now 21y and a perfectly happy, well adjusted young women. Yes. She had phases of wanting a sibling but overall has loved her childhood experience as a single child. She's never been lovely - she is so sociable and friendly, makes friends easily, so it rarely left to her own devices.
We didn't plan it that way so had to get over the disappointment of being unable to have further children. But our live has been so happy as a family of three we now wouldn't change a thing.