Your DS has at least two great options, OP.
Historically I’ve been wary of degrees like the AI Eng degree but I think it really comes down to a question of taste. The Data Science degree is somewhat similar and I hesitated over that, but it is proving very successful.
I think the AI Eng degree is very likely to lead to good employment and to be pretty interesting as well. The Maths is likely to be in concentrated (relative to high school Maths) in the areas of Statistics and Decision Maths. In a course like this that is pulled together from different historical disciplines, there may be less of a sense of getting to grips with first principles. Some students may find this problematic (I would have) but most will not.
We don’t have a sense yet of what the advanced training and the career path will look like, but getting into the field early is likely to create good opportunities.
I trust Bristol to develop a high quality programme.
If DS loves Physics and the Maths associated to it, he may well prefer Mech Eng. As @TizerorFizz (and possibly other PP) has said, Sheffield is tops. The degree will never go out of style and the career path is proven.
If DS listens to his intuition I think one of these will seem more appealing than the other. That is the programme he will engage most fully with, and wholehearted engagement is the key to his best future.
He is in the fortunate position where doing exactly what he wants is the best option. Congratulation to the lucky guy.