All I can say is maybe you can send him cards, like one for his birthday, holidays, etc. There's always stuff between them that you don't know about. I'm sure she did what was right for her. It's sad, and you're a good hearted person.
I know it's very hard to let go of someone you have deep feelings for. But maybe we need to think about what's best for your son and the ex girlfriend. The best thing for your son is to support him . Obviously something wasn't working and nobody outside of your son will know the whole story...
I've been there myself and my parents felt bad for him!
Think of the positive. It's much better it happens now rather than in 20 years and 3 kids later.