@Siriusmuggle RNCM and TLCMD would be my top 2 recommendations if someone asked me. It depends a bit on the instrument, of course, but both are such nurturing institutions with a very high level of teaching, despite the 'worse' reputation. Trinity was/is #1 in the UK for my instrument and a few other courses, but due to historical bad reputation people still brush it off automatically, against other colleges.
They are a lot more relaxed and there is a tendency to push students towards playing or doing what they enjoy the most, and develop their own unique portfolio career, instead of the majority all learning X piece in first year and YZ excerpts and concerto in second year, etc. It's a bit more informal and free or flexible. This observation comes from my personal experience, both on the student and the teaching side, at many of the UK conservatoires.
Going to RCM/RAM brings some extra 'prestige' with it, but it very much depends on the personality of the student, and most importantly, the class and department they will be a part of.
Don't feel like it would be a bad move to reject offers from the more prestigious conservatoires. He should go with his preferred option and I'm guessing there are good reasons why RNCM is his top choice!