Seeking expert testimony is a sign of strength. A personal relationship with a caring professional is proven clinically effective.
I believe that I can help.
You and your friend clearly have a good relationship. If he has been looking at rings then he has been considering formalizing the relationship with marriage.
His previous relationship ended many years ago, but perhaps on a sour note, which left him feeling anxious whenever he considers re-marrying.
He may be afraid of failure and may feel more secure knowing that he is not bound by civil law (and the church if you are thinking of a religious based marriage). He may be thinking, "If it ain't broke, don't fix it", as if getting married might somehow jinx the relationship.
He would like to think that he could be free to come and go (even though he is already faithful and committed to you). Men often feel this way more than women do.
I suggest that you address the question to him, and tell him that you love him and want to actual want to be his wife because it would unexplicably but absolutely would make you feel better - to be able to say (even to yourself) that you are his wife and he is your husband.,
Tell him that you understand how anxious it makes him.
If he has children there may be considerations of property rights that are bothering him because he does not want them to lose out on his inheritance. If he does indeed have children, then you can talk about a pre-nuptial agreement which can ease his apprehension.
If this is not the case then he just has fears that he may be able to overcome.
Let me recommend three excellent books that can help you very much:
You will have to approach this gradually as you certainly don't want to scare him away (although he seems that he is there for the long haul).
Learn what you can (and there is a lot to learn in those books) and I believe that you will eventually be able to insert an "r" between the M and s (Ms. to Mrs.).
I wish you great success and shall keep you in my prayers.
Elliott, MAE, LPCC, NCC, CCMHC