I agree with prior responses. Stop for a moment and consider that your child is "only" three years ahead. Since you asked, yes, he could stay in the first grade class, but he's not going to learn anything in math. Is that right? I would also, personally, keep a close eye on the reading. In other words, yes, you may be comparing your two sons and not looking at your younger one versus his age peers.
Did you have a difficult experience getting accommodations with your older son? If so, I can see why the notion of doing that again doesn't sound appealing. Yet, your DS5 is his own person, and deserves what is right for him, too. From the results and the tester's recommendation, you have a very strong case for differentiation of significant nature. And, it may actually be easier since it won't be such a surprise as your first son may have been to the school. I know that in our case, with a PG older son, the school is already commenting that they are prepared to accelerate our younger child if needed! The work we did earlier is helping (it also may have helped another child, who was accelerated by a year last school year).
Good luck!
