I believe the most important thing for you now is to clarify (or confirm) whether if a medical student who dioes NOT have British passport can get internship (houseman) place or at least have an idea of the chance of getting an internship place e.g. 20% or higher? coz if your son cannot get in internship , he cannot get registered in UK, then it would NOT be possible for him to even apply for the Licentiate exam in HK (coz the applicant has to be a registered doctor abroad).
UK internship prioritises UK local students, but then still UK local students find it difficult to obtain the internship, not to mention overseas. First priority is UK local students, then EU students, then overseas. By the time overseas students get to choose, the popular departments will be full. The chance of getting an internship for HK students is very likely less than 1/3.