I don't see marriage happening to this fellow anyway. You do not have any of the classical indications for marriage with this person.
The classical indications for marriage include person A's sun conjunct B's moon, A's sun conjunct B's descendant, A's venus conjunct B's Descendant, A's venus conjunct B's Ascendant, A's moon conjunct B's Saturn.
There are other indicators for marriage.
I do see your sun conjunct his Ascendant. This is more typically an example of you are what he aspires to be. This is an indicator for friendship, not romance or marriage. I also don't see many positive connections between your two natal horoscopes.
On a more practical note, if he really wanted to marry you, he'd have asked in spite of his parents. And you haven't stated what YOUR parents would think of such a match. I don't know what wearing a diamond will do for you, but I think it is more likely to fail to give you the results you are hoping for.
And after 4 and 1/2 years, if he wanted to marry you, he'd have indicated a more decisive commitment by now. Stop chasing him.
He is not the right one for you.