Why Won't He Propose?

My SO and I have been together 3 years. We have an 18 month old boy. We live together, our finances are aggregated to use money and pay bills. Despite these few 3 years, he and I have done a lot ad a couple which tends to make others think he and I have been together much longer. When he introduces me to people or talks about me to others, he always talks about me as his wife or addresses me as Mrs. Scott.

He and I have discussed marriage and both desire it. We have even looked at rings and discussed ring/wedding budgets. But despite all of this, he has yet to propose to me and just days that "he will."

What does it all mean? Why won't he propose already?