How old are they?
how long were they dating?
How long was he in love with her before they became a couple?
Did she always feel the same about him? If so, why did it take so long for them to become a couple?
Sometimes when we want something so badly, that when we finally get it, it's just not everything we thought it would be. Perhaps he built her up so far in his mind, or built up being in a relationship, so much in his mind that it was a sort of letdown when it finally happened. Also, it definitely could be the fact of him going to school, perhaps she proves to be too much of a distraction for him and he would prefer to focus so that he can pass his class.
Personally, I think it's very big of him to make the decision to break things off, rather than stay in the relationship if his heart isn't in it. He deserves respect for making a mature choice that most men would avoid these days. I understand that you love her, and there's nothing that says that you cannot remain friends with her as long as it does not interfere in your sons life. There may be one or two or there may be many women that come into his life who are amazing, and that you will really like, but the fact is, it's his choice and ultimately you have to respect that he will make choices that make sense to him, and work for his life.
I welcome your thoughts, let me know if you want to talk more