Congratulations to your son @Siriusmuggle - having a choice is great, and also so difficult in a way!
On your ‘how to choose’ question, my top recommendation is to have as many consultation lessons as he can get/you can afford, if the conservatoires are offering them. Email admissions if they don’t offer any and ask. It’s incredibly important to ‘click’ with the teacher or at least have an idea of how they teach and whether he’d be happy being a part of their class. After that it comes down to scholarships (if any), performance opportunities, chamber, variety of courses and ensembles, campus, distance from home, accommodation… and all the ‘normal’ uni usuals. But really, teacher choice would be number one in my list of priorities. It’s someone he’ll be spending the next 4 years with, so it’s an important choice.
@gogohm I had many colleagues during my postgrads coming from universities, so as you point out it’s also a valid path for those without conservatoire offers. Many uni students apply for a masters after their BA and focus on performance for 2 years before deciding if a performing career is realistic or not. They’ll have that extra - prestigious - degree and experience regardless.
In general, each conservatoire holds auditions for the same instrument and/or department over several days-couple of weeks, so the long wait is completely normal. Nail biting but normal! My (London) college doesn’t usually release results until mid-December, especially if you are a candidate for a scholarship, as those have to be discussed and awarded after everybody - from all departments - has auditioned.