Relationship Progression?

So we have been together for two years. We bought a flat on our one year anniversary but he doesn’t seem to want to progress. I am 22 and he is 24 so we are young. However, I am ready for the next step, we are more mature for our age than most people we know. I have PCOS and want to have kids (if I can) when I’m younger rather than older. He wants kids but said he would consider adopting if I can’t have them. Everyone is joking to him and asking when he’s going to propose and he’s told me that it stresses him about out! I’d say yes in a heartbeat and I have an idea is when I’d like to marry him. Why doesn’t he want to propose?