Maths is an excellent subject and can lead to many careers - academic, industrial maths, banking, science, business. Most graduate programmes, in any field, for example, want numerate analytical graduates, which basically means Science, Tech, Engineering, Maths. So a maths degree or other STEM degree is one of the most career-useful subjects to study.
Someone with a maths degree from a good university is going to have a lot of opportunities, and it really won't restrict him.
There are also other STEM degrees which require a lot of maths - the multiple types of engineering, from civil, mechanical, through to electronics (which will lead to well paid 'tech' jobs), physics degrees, geophysics etc. Does he prefer theoretical maths, or applied maths. If he likes theory then I'd probably look more at maths and physics degrees, if he likes applying in the real world and problem solving, then look at engineering, geophysics etc. Statistics-heavy courses are also popular with business, finance, civil service etc.
Economics - I would probably only do economics at a top tier university.