Veragrace, I feel for you, as well. (((Hugs!))) Since I frequently watch my grands, I know how involved a "granny nanny" can become. I don't blame you for speaking up when your son spanked or yelled at your GS. It's still not unusual for parents to use these methods, of course, but if GS was turning to you about these things, how could you not say something? My heart aches just thinking about it!
Maybe you should have respected your son's wishes on other issues like how much to feed GS (unless what son wanted was inadequate). In fact, I guess I think so. After all, GS is HIS child, NOT YOURS. (That's the hard part, I think, about helping to raise a grand - you -general you - may feel like their parent - but you aren't.) But yelling and hitting are a different matter.
Also, I imagine it must be have been heart-wrenching to go from taking care of GS every day to only seeing him on holidays! Such a HUGE change! Oh! I'm glad you're able to keep in regular touch with him by telephone, however. Perhaps, when he's older, if his parents will allow it, you can also talk to him on FB and so forth.
But what I really don't understand is how parents can think it's ok to separate a child so much from someone who has been so much a part of his life! I'm not surprised that GS misses you and they shouldn't be either. Poor little guy!
I'm glad for his sake, though, that his frustration about that seems to have lessened - going from "I need to go back to my Nan" to "Why are you keeping me away from my Nan?" I hope, in time, again for his sake, it will simply be, "I wanna see my Nan" or "Can we visit Nan?" But, happily, it's clear you'll always have a central place in his heart. (As he gets older, he may be able to visit you on his own whenever he wants, though, of course, he may be too busy with other interests and activities.)
Meanwhile, I'm sorry you're relationship with your son seems to be over. I can see why, however, and don't blame you. I'm not sure how the visits with GS are continuing if you "have nothing to do with" your son though. If you don't mind my asking, how does that work? Do GS and his mother come over and visit without your son? Or am I being too nosy?