When someone stops loving someone as his girlfriend stopped loving him, thre is nothing one can do - love can not be reignited once the flame burns out - but come to accept this as envitable................. no one can do this for you except you yourself, or in this case your son............. if it is his own fault , then he has to accept this and move on, forgiveness being only possible within himself not from a girlfriend who does not want him anyomre.
Its sad, its hurts, but he knows that he caused it and perhaps what he needs to consider is why he caused it and what that means.............deep perhaps, but he needs to delve into himself and consider why this has happened, with honesty ................
You can not make him feel better, it is not your job to do so, you are there to guide him thro the process of forgiveness within himself, allowing him to find a new path................ perhaps hopefully a more honest path................ you need to be strong and see this not as you healing him but aiding him healing himself
Good luck Px