In answer to some of your questions:
As far as I know he has not been told how to use the extra time.
I don’t know enough about dyslexia to understand what specific provision would help him in managing his dyslexia. I am not an expert and so was looking to the SENCO for that expert input.
I don’t think it is about working harder but about working smarter. Understanding his own limitations and having tools to help mitigate those limitations. Teachers confirm he has the knowledge as is evidenced by his verbal contributions which would suggest he is learning and retaining the information but is not able to replicate this in writing or typing in some (not all) exam scenarios. I think stress makes things worse.
Colour paper doesn’t work for every dyslexic and I have no idea if it would make any difference to him. I was just using this as an example of someone who is dyslexic and has tools tailored to their particular needs. This is why I would like the expert (SENCO) to help him (and his teachers) understand how his dyslexia impacts in exam scenarios.
I think the biggest problem with a laptop/keyboard is that he has to hop between paper (diagrams) and the keyboard which must be confusing.
We have not seen the latest dyslexia assessment and so are not aware of what it says (I have asked) about how it presents in him and what might help him manage it.
His dyslexia may not be the issue in exams (although I suspect it is a big contributor) but without expert analysis of some of his test papers we will never know.
I have raised all of the above at various times with teachers and the SENCO but nothing ever seems to happen. Thanks again for your suggestions.