First, I don't think that your son will be damaged if he tries 5th grade math and it doesn't work out, provided that you don't leave him for months in the situation if it ends up turning toxic. Given that you are already thinking ahead about this, I don't think that will happen.
Who were the people involved in making this decision and were they supportive? If the principal was supportive of the move, perhaps you can talk, maybe informally, about your concerns with the 5th grade math teacher. Having that math teacher at least receptive to the idea is definitely preferable.
Do you sense that the teacher was downright hostile to the idea of your DS being in her math class, or is she just highly doubtful that it is going to work? She probably has never seen a child like yours before and can't imagine that this placement could actually work, but it could be an opportunity for her eyes to be opened. I know that happened with my DD. The teachers didn't think the acceleration would work, but within a few weeks their minds changed and, not only that, the minds of many of the teachers in the school were opened up a bit to the possibilities.
About EDM, I don't think that the writing requirements are extensive at any level, including 5th grade. Virtually all the homework is worksheets on which you don't even have to write the units down. This would not be a concern of mine at all.
As far as adolescent talk, sure, it might happen. Hopefully it won't happen a lot since your DS is only going to be in there for math and, hopefully, the kids will be focused on math and not chatting. Still, even if it does come up, it is a good opportunity for you to open lines of communication with your DS and have discussions about how your family feels about certain topics. It's not like you are dropping him on an island with preteens and leaving him to his own devices!
And finally, if your DS is ready to be accelerated three grades in math, you should have no doubt that he has the mental capacity to master all the various methods of multiplication. He is probably even smart enough to realize how silly it is to have to solve the same multiplication problem three different ways!
