If he's 27 years old or younger, he may just not be ready for anyone right now. If you've been together for less than a year then it's too soon to know. If you have been together for more than two years then it's probably a good idea to cut him loose if your goal is marriage.
It can mean many different things. He may mean that he hasn't got to know you well enough or doesn't want to make that prediction. It could also mean that he's growin away from u.
What does he mean when he says he is not sure I am the one?
I just want to understand what a man means when he says he is not sure you are the one he wants to marry