IMO, he needs to sign over educational guardianship to you. I do know he'll need to be 18 & an adult to sign this sort of paperwork. (You can't do it ahead of time but you can have the paperwork ready for his signature on his birthday.) A FERPA waiver might also be expected by the school. Many special ed attorneys offer a free 15 minute consult but they will likely want to set up the paperwork for this if you contact them. (I'm in PA & the age of majority for IEPs seems to be 21 here so I didn't have to deal with this.) Not sure if you can find a free template for this online.
How do you know this will be his last year? He can stay in school until 21.
What are his post HS graduation plans? Will he need a current evaluation to get accommodations where he plans to be post-graduation? If yes, ask the school to do an eval during the 2024-25 school year so you don't need to pay for one.
Also, every child should be providing their parent with POA when they turn 18. Disability/incapacitation can happen in an instant. I remember a lawyer with 2 children had a POA set up for his disabled adult child. It was his typical adult child who ended up hospitalized from a skiing accident & he couldn't get any info from the hospital.