I'm not sure, how old your son is now?
Children start being more social from 6 onwards, boys around 7.
1) Considering that at IMS Elementary, they are often put into small groups to work on projects, I would say Montessori is ideal for 'fostering' friendships between students. There are at least 20 students in each IMS Elementary class. Will your son take the school bus? I've noticed the children chat with other kids NOT in their actual class, this is another 'social' opportunity.
2) That IS true, that there are slightly less boys, I know one friend who moved her son from IMS to ISF, for this one reason. She also told me that (at ISF) the Chinese is about the same level but the standard of English and mathematics are FAR behind what they were even learning in Casa.
3) I would discuss this with IMS, the great thing is, they listen to you and take your needs into consideration.
Nothing in life is perfect and there is no perfect circumstance.
IMS is really expensive but I believe my daughters are getting an amazing education, besides the amazing Putonghua & mathematics, they are learning self-motivation, confidence and to think for themselves.
none of the other "Montessori" schools in HK are credited by Proper Montessori Authorities (IMS has IMEF and IMC), this means IMS delivers a true Montessori education with high academic standards. IMS is the only school in Greater China to have the "IMC" accreditation.
Good luck with your difficult decision!