Mathematics - for university application

[newtothis15]

My ds in year 11 started really liking math and wants to apply to uni for maths

Is there anything outside school he can do to enhance him? Learning/ competitions?

bmos.ukmt.org.uk

find a maths circle, e.g.
https://mesme.org/maths-circles/
or
https://www.londonsmp.co.uk/

look for extracurricular competitions / courses
https://www.livmathssoc.org.uk/cgi-bin/sews.py?OpenChallenge
or
https://www.coursera.org/learn/mathematical-thinking

contact UKMT about mentoring
https://ukmt.org.uk/what-we-do/about-ukmt

look for university outreach opportunities
https://www.maths.ox.ac.uk/outreach/oxford-online-maths-club

When considering school options:
for Y11, if it's offered, try to take further maths (at GCSE or equivalent - there are level 2 / level 3 certificates or IGCSE's) gives a head start on A level material.
for Y12/Y13, try to attend a school or 6th form that offers further maths A level and strongly consider doing double maths A level. The top-ranked 6-8 unis for maths either require or prefer further maths A level, and expect it to have been taken if the school offers it. Physics and comp sci A levels are also complementary but not required.
(You can, of course, do a maths degree only having done standard GCSE maths and single A level maths, but you'll be less well prepared, and the top half-dozen unis will be out of reach, so your options will be fewer though they'd still include some good unis with strong maths depts).

When considering uni options, be aware that several unis require additional exams (other than A level) to be taken, some in early year 13. Ask schools / sixth forms what support they can provide for MAT, TMUA and STEP.
If the answer is "none", have a look at AMSP which runs some support courses
https://amsp.org.uk/. (Again, this mainly applies to the top half-dozen unis for maths - there are many other unis / depts that only require A level or equivalent, but if someone is aiming high, they need to be aware that an early start and additional preparation are required for some unis).