I really don't think any of the suggestions about keeping in touch/Skyping/postcards etc will completely solve your GSs problem. His problems are probably more linked to what has happened in his past than just your trip. You say you looked after him since he was 10 months old, possibly when his mother went back to work? Anyway his first experience of separation, his parents are divorced, lots of children blame themselves for parents splitting up, no matter how young they are when it happens. His parents have both remarried, another instance of feeling a bit left out. He has suffered a lot of emotional damage in his short life and your trip may be just a trigger bringing out all his uncertainties and feelings of loss. He needs everyone in his life to talk to him about what has happened to assure him that he wasn't responsible for the divorce, that both his parents still love and care for him, that he has a special place in both their families and that you will always care for him. Even then it may take a long time to help him through this.