D.K.
Marriage shouldn't be entered in lightly as I am sure you know. First and foremost you need to think of your son. If this man has cold feet and your son is so attached that can cause him a lot of hardship if the relationship ends. You need to sit down and discuss that you either need to move forward but aren't going to stand in the same place. Relationships at the three year mark should either be headed forward or be done with in my opininon. You cannot rush him though and cannot force him into anything he isn't ready, it would be disaster for you and your son if he felt trapped into having to do this. Do not give ultimatiums, but word it as such that you need to get on with your life and if he isn't ready for marriage that is fine, however you have to consider a good solid future for your son and need to move on.
Don't be upset, men sometimes drag their feet, however if you continue to allow it, it could go on for more years. You have to do what is best for your son.