I did not see the Bath thread. However when I was a STEM admissions tutor in our highly ranked School, I dealt with external transfer students. Typically they leave their first university after Y1 and wish to start in Y2, but lack some necessary background. On the rare occasion that the gaps seem minimal enough to approve the direct transfer to Y2, they usually struggle. It is much sounder to start again in Y1.
None of this is about the external nature of the transfer. Gaps are gaps. Students (and their parents) do not tend to appreciate this until they are floundering.
Based on the descriptions here, I would not be put off Bath by that thread.
It’s true as PP have said, importantly, that some choose maths intensive subjects because they love maths and are up for a challenge whilst others fall into these areas because maths has always come easily. OP, the fact that DS’ school suggests he aim high means that they think he is up for a challenge.
The Schools of Maths likely to give him one are not necessarily the most ‘nurturing’, but within any of them it is usually - not always - the case that personal tutors, lecturers, project supervisors etc respond well to engaged undergraduates, and do make time to help them. (This is in addition to the Student Support Services, formal mitigations, etc, for students with diagnosed disabilities)
In particular although it isn’t something that can be mandated, a good relationship with one’s PT can be a great source of moral support. Students can cultivate that by fulfilling their side of the relationship as laid out by the university, which a shocking number do not.
I suggest DS join the online forum The Student Room. He can pose any questions he likes directly to Maths students (in particular degree programmes, if he wishes) at whichever universities he likes. The quality of the responses is usually very good.