well putting aside he is going to be there with a gaggle of private school gals who are there primarily for the sloaney networking / social life
Courtauld is without a doubt the best place to study but it has limited opportunities for a social life. It is not part of University of London so a bar and some yoga but a lot of people who are passionate about art studying it in amazing surroundings and part of a world class art gallery. It will still be there for postgrad though if by that time he really wants to pursue a career in art.
UCL, excellently regarded course and you get the benefits of being in the capital and the benefits of a more varied social life if he wants to pursue other cultural (and social) opportunities.
Glasgow obviously is a centre of excellence for art, you would need to research the fees issues.
It partly comes down to the emphasis of the courses, some are more theoretical, some lean more towards the vocational curating side, some more focused on modern art, some more on traditional.
If he is seeking employment in the arts, get work experience, lots of it, and network, network, network. Plenty of opportunities outside the stereotype of sloaney girls manning reception in Daddy friend's gallery but also a good history of art degree is not as looked down on as some mumsnetter's might judge in the wider world of employment if it comes from, especially UCL.