My experience isn't up to date although I still work in an applied maths area. However I would caution that since many of my course mates hit their ceiling in maths and ended up with 3rd class degrees (despite great results in 1st and 2nd year of a four year degree) I'd advise you to look at the options available in later years of the degree just in case (probably doesn't apply to those who ace STEP).
Clerk I’m personally in awe of anyone who does a Maths degree. For boring and complicated reasons, I had to do various mid level Maths courses at uni, my absolute favourite being Discrete Math. (God, I loved that class! Barely passed it, but loved it. 
) I was in awe of everyone who just took it all in and “got it”.
I’m hoping DS is mathematically inclined enough to get through a Maths degree. He only messed up one part of one question on his Higher- which he realised as he was walking out of the exam! The same happened on his Nat5 so he’s kicking himself again.
He lives and breathes Maths. He started Higher Maths thinking, “shit, this is hard”, spent the October break cramming as much into his head as he could, getting a bit ahead of the class. His teacher then gave him harder and much more interesting work which he liked, so he went through the rest of the Higher curriculum and was done by Christmas. Before the summer holidays, his Maths teacher asked him to please not teach himself AH Maths over the summer break.
I’m hoping this means he has enough interest and natural ability to do a Maths degree. I don’t have that ability so it’s hard to judge!