My 10c worth: IMHO you owe it to your children to at least open up a channel of communication with your H - because not having one can really be detrimental when you need it. S20 might be a little hurt right now - my heart went out to him when I read that his father basically abandoned him in favour of a friend. Having said that, we don't really know if his father offered a place at the Christmas table as well, or if everybody assumed that S20 would actually be happier to spend Christmas with you. That is probably something S20 needs to answer.
Anyway, the fact that your H is also discussing S20's 21st present like you two are parents who work together on some issues around the children is interesting. My XH has made it very, very clear that he wants his relationship with our boys to absolutely exclude me. Any time I have tried to work with XH on something that involves us both (e.g. setting up S21 at Uni or buying S18's 18th birthday present), XH says one thing to me and then tells the boys that I am being obstructive, critical, controlling yada yada yada. What he doesn't know is that the boys let me know there are supposedly issues arising on my side when they take me to task - and I set them straight about what I understood the arrangements to be.
Stupid thing is I keep all texts and emails (I, like OwnIt, know that they may eventually be needed in court) so I can prove XH is lying to them (I haven't had to yet - my word is enough - but they know I can). Eventually they will work out for themselves that their father's word isn't always trustworthy, and when that happens, I suspect the relationship dynamics might change a little.
In any case I've been forced to abandon any hope of a workable relationship with XH. If I could though, I would much prefer a civil, amicable relationship based on trust and respect. If that's what you think you've got (in relation to the kids) I would say go for it! Just be sure to manage your expectations so that you don't go extrapolating a good parenting relationship into something else.
