Oh, GagaJo, my heart is aching for you and your little GS. For him, I imagine it's the same as having parents divorce. I doubt he knows the difference between 6 weeks and 1, etc., but when you come back, it must mean a lot to him. And then when you leave again after a "fantastic week," I can see where it would be very hard for him. Poor little guy!
IMO, you have been given some wonderful suggestions. And I'm glad that skyping helps. I also think it might be a good idea for his mum to plan a fun event or two for him for after you leave and/or the next day to help distract him from his sense of anxiety. Perhaps you could suggest it though, in the end, it's her call.