As an ex-teacher and not knowing the fully back story except what I've read on this thread, I would say the mental health wellbeing will far outweigh whatever he will learn in school in those 3 days that he is not in school, especially as a bright pupil. It is a nonsense to say that is just another burden for the teacher. The amount kids learn at that age in 3 days by the time you have breaks taken out, moving from class to class, getting the kids settled down, etc, is actually quite small. Most decent schools have a lot of the teaching on the children's learning space so it would be perfectly possible to avoid putting any pressure on the teacher and other children are usually happy to help each other. One thing I wouldn't do is to encourage a child to lie. I wouldn't discuss the "rule breaking" with them either so the boundaries aren't blurred.
We had a similar circumstance when we lost our daughter. One school was understanding, the other said no way to the holiday our family had bought for us so we could heal together. I just wrote and told them that I was sorry but in these circumstances we were going. The children had to keep a diary, do currency changes, they learned to swim and we went to museums. The child whose school refused learned more in that time away than he ever would have in school; quite frankly they failed him at every step but that is an entirely different story.
I hope your Grandson has a lovely, bonding time.