IS (ie ESF) or local DSS (ie DBSPD/ St. Paul's Co-ed)

Mighty

My input is:
(1) your son will never turn into gweizai just because he is studying in IS.  Dont forget here is Hong Kong and the culture is still chinese.  Even if you want him to turn into a gweizai, no way hosay.
(2) I think 90% of the IS students go to Chinese classes after school, be it PTH or Cantonese.   So that will compensate on the chinese side.
(3) One of my friends, husband : local Hong Kong (educated overseas), wife : PTH native speaker.  Their son uses English and PTH respectively with them and his level is native at both languages.
If money is not an issue, I am sure students going IS are far more happier than those in local schools.