I totally undertsand your reasons for the grammar.
I think he is suffering partly from the stress of year 7 and the difficulty of making new friends. That would be exactly the same at the local school, but it will be impossible to convince him of that.
I think you need to acknowledge how hard it is for him.
So I would sit down with him and make a plan.
Talk about the hard things, make a list, try and get things on the list that he might not think about, eg going to different lesson for each class. If he is ASD /ADHD he will find that chopping and changing and new teachers really hard. Get him to think about which of these would also be hard at the local school, and which ones might be easier.
Also think abotu if anything else might be harder at the local school.
Try not to be pushy in your view. Make it clear at the beginning that you are both going to explore the options.
You should be left with a load of stuff which will be difficult anywhere, but some specific things which are harder at this school, including travel, which is really hard on year 7s (mine all travelled 1 hour door to door and so I get it)
Then think about a plan. I would try and get him to agree to 6 months in the school, with a review every month. So at the end of every month you sit down and see how it is going and go over the list again.
Be willing to work with him, because if you can't get him onside then he will crash, but also you are the parent so helping him to try and look at the big picture.
It is hard. My son hated that we sent him to grammar, I promised that he could move at the end of year 7 if he was unhappy. He opted (reluctantly) to stay.
At age 17 he told be that he was really grateful that we had sent him there, he needed the competitive and academic environment. He finally understood why. But that is not the case with all kids.