It's tough, and the best ingrediant to getting over someone, in my mind, is TIME. It's a hard pill to swallow, for sure.
Also, if someone has a strong connection with another, I think it can be really devastating to lose that love in our life. I think modern, industrialized cultures don't place enough importance on grieving. And, I think we trivialize the loss of a love.
So, I think, for your son, if he can realize that grieving the loss of a girlfriend is like when someone dies who we're close to, maybe he can deal with it more head-on. I don't know, but it seems to me, that whenever I break up, the tendency is to deny what I'm really feeling. Alternately, there comes a time to pick up and move on...
Depression can set in when we turn our sadness and/or anger inward and don't release it. I think it's important to be brave, face our feelings head-on, release some emotions (without wallowing in them), heal, and move on.
I like the ideas of doing some good service to feel better about ourselves, volunteer or whatnot. Also, I think you could find some good books on these issues to help you and your son.
Best wishes, Please keep us posted as to how it turns out.
P.S. I like people who deem their lovers as important to them and it sounds like your son is like that, and saw her as precious in his life. A lot of people out there jump around from lover to lover and don't really care that deeply. I think that sensitivity is a gift that allows us to open up to the subtleties and depth of emotion.