That's great that your DS is on board. It helps that he's close to age of the kids in the receiving class. Maybe you can use that as part of your explanation to the mother of DS's friend? Along with explaining that the school thought it was the best decision based on evaluations they did? Or that DS already knew the material that they would be teaching in 1st?
If it were me, I would emphasize that your DS still wants to keep the friendship alive, and ask if you can set up regular playdates or sleepovers, etc. Will your DS be able to be in any outside school activities with his friend, like cub scouts?
My son skipped 1st, and remains friends with a boy who is a grade lower. We just set up as many play dates as possible.
