I feel for your son, he is trying to do his best but it seems that whilst he loves her she no longer loves him as much as she did and she wants different things. Sometimes relationships are unequal and some people are more in love than others. Your son has been so busy working and travelling he probably didn't notice signs of cooling off so this is a horrible shock. She may have been unhappy for a while and this has been brewing for a while. Long distance relationships are vulnerable.
He could try counselling but for it to work both of them do need to want to try. If she refuses to give it a go he may have to walk away which is easier said than done. If they are talking that is good but your son also needs to listen carefully to what she says so he can understand why she feels as she does.