I found your comment quite offensive too @Popeyeandolive. My eldest child is diagnosed ADHD, my youngest isn't yet but no doubt will be and I suspect it runs down from me.
I quite perfectly well know how to use manners and so do my kids, suggesting that ADHD is the reason people don't use manners is offensive.
It sounds to me that this child may have had a rough upbringing and not been taught basic societal expectations, just remind him when he is with you that you expect please and thank you.
No need to be upset that he asked you for a drink, when other people's kids are with me they are treated as part of the family, as such, they are welcome to ask and I am able to respond yes or no, same as with my own kids.
I also disagree that children can't change, depending on the reason the bullying occurred, the child may well still have a bit of a smart mouth on him but be a kind person inside who just hasn't been raised right.
If the not saying thank you is your sole reason for cutting contact I think it's a silly reason and you should consider it carefully.