Ugh.
That's all I've got, really. Ugh.
The guy is a jerk, clearly. I'm with delbows: I probably would have posted it on the classroom door. If the other parents agree with what he says (fat chance!), then no harm done, right?
If not, the hue and cry would have been very fulfilling! But then, I'm no diplomat in these matters. I can't stand bullies, and that's what this guy is.
This is the new skip, right? That you just got? Just want to make sure that I'm not steering you wrong. If I'm remembering wrongly and this skip happened at the start of the year, then please disregard the following...
The question of the dad's social clout is the big issue, I think. Is he likely to have the means and/or the opportunity to make trouble for you this year?
If not, then hold your head up and smile charmingly at him when you see him. He'll hate that!
You and your child have every right to be in that classroom. Don't let this nutter poison this well for you, too. He doesn't have the right.
If you think he can and potentially will make trouble, and if the teacher is sympathetic (as she seems to be, no?), then I think I would ask to meet with her, show her the letter, and then tell her as matter-of-factly as possible that there is an uncomfortable history between the two families, as evidenced by the letter, that you thought she should be aware of. Make it clear that you merely want her to know about the issue, not that you need to have any action taken. But that you hope that she'll be on the lookout for any otherwise unexpected problems.
I'd end with expressing gratitude to her for welcoming your child so thoroughly into the class and tell her how happy your DS is there.
Now, I'm really bad at this sort of thing. But that seems the prudent course of action to me. Whatever you decide to do, I think you can't let this rotten person ruin your DS's situation. Don't let your discomfort with this man or his child mess up what's working!
A confident smile will take you a long way in this sort of situation. If necessary, fake it! He won't know the difference!
{{Hugs}}