Most local schools teach Chinese in Cantonese with some teach in Mandarin leading upto HKDSE. HK Exam Board allows simplified Chinese or a mixture of simplified and traditional chinese when the students write.Oral exam including presentation or discussion of topics can be in Mandarin when the student take HKDSE. But exam papers and instructions are in printed traditional chinese. Never saw books for the local Chinese courses in simplified Chinese.
The famous schools you mentioned should be teaching in mostly Cantonese . Chinese teachers and fellow students should speak enough Mandarin so your son will not have problems talking with them.
HK students learn difficult Chinese including classical Chinese texts from Secondary 1. They normally also have compulsory Chinese History courses taught in Chinese from Form 1 to Form 3.
If you do IB or other countries' Chinese, will be in simplified and Mandarin. But these overseas courses are not subsidised by HK gov.