@PocketSand I think you are doing an amazing job with your son. This post is a bit long but I hope you read it and dont repeat my struggles mistakes
He sounds very similiar to my son, eg great Maths skills, not writing enough discussion, losing marks even when he knows the subject, misunderstanding critical emails, missing hand in dates, not aware how much help he needs, always slightly immature compared to peers etc. I helped him revise, spent ages liaising with disability support, and he eventually got an average BEng degree, but nowadays thats not enough to get an engineering job, so the whole thing was a very expensive mistake.
Engineeing courses seem to require an above average maturity and self-organisation. It will only get harder in second and third year, with less and less new maths involved. The masters year is mainly subjects like law and project management, with a long thesis requiring lots of discussion.
Perhaps, instead of struggling to overcome things he finds difficult, he could take a step back and focus on his strong point, Maths.
From what you have said he wants to do engineering, but you recognise he'd be better doing Maths. You might need to insist, let him know you are immensely proud of him for the effort he has put in, ( I would be!) but it turns out that a maths degree would suit his skills better. There will be more interesting varied maths, fewer lectures, probably no labs, less discussions to write, more free time to rest and enjoy life. There are joint courses where he could do another subject with maths if that is of interest.
Maybe you could visit the potential maths department, to see what the course involved, and the fascinating jobs a maths degree can lead to, including many with software and analysis, so similar to the bits of engineering he enjoys and got good marks.
It may be difficult for ASD person to change, but it would be worth it in the long run, I wish I had known.