Hi, OP -
STEM academic here.
I agree with the university staff who have responded. The extra time and other supports DS will be entitled to will simply level the playing field for him. In these brutally competitive times, why would he not take that opportunity?
When tutees disclose to me and are of two minds about accepting these mitigations, this is how I put it to them. Of course I emphasise that the decision is theirs, but they usually end up seeing it this way.
I think your question about future employers is excellent. DS can use his university years to continue learning to play to his strengths as much as possible, and it is sensible to go for jobs that do so, as well. But he is likely to face interview rounds assessing his communications skills, and this may include writing or preparing presentations under pressure of time. So he needs to work on this side of things now as well: if he buries his head in the sand, it will come back to bite him.
Obviously DS will need to reply honestly to any verbal or, especially, written questions about his disability but I think it is prudent not to bring it up unless and until necessary. Then be matter of fact. Many brilliant scientists and engineers are dyslexic so it should not be a big shock. The more he has learned to cope, the better. Ultimately a decent employer should genuinely support an otherwise satisfactory employee, but we hear of too many exceptions nowadays compared to the past.
I suggest that DS sign up with the Office for Students, whatever it may be called, at his uni. If he stays in contact with them they will tweak his mitigation plan as needed; they probably have ideas for approaching employers; they may advocate in case of a Mitigating Circumstances issue, etc. But they can only act on what they know; a good working relationship, which doesn’t require much, goes a long way.
Best wishes to DS.