Men and proposing

My partner is 32 and I am 23 we have been together for nearly 3 years. He has bought my engagement ring 6 months ago but he won't ask me. He says he is waiting but doesn't know what for or why. He has been engaged before and had a child and she cheated and left but he says that doesn't come into it. We talk about it often by we I mean I bring it up and he listens (very frustrating) my question is will he propose and how long is too long to wait? I also suffer with PCOS and currently cannot fall pregnant I really want kids although we do have sex he wants to wait till we are married. What do I do???