A few notes:
Taipei - It meets your other requirements, but maybe not the best for diving. It takes 5-7 hours to get to the best sites. There is at least one local option though: (from
An Underwater Photographer’s Guide to Taiwan which also covers the better sites to the south)
Longdong bay is an hour’s drive from Taipei on the northeast cost. The dive season here lasts from May to September. In the springtime, the ocean can be a bit chilly—23–25ºC (~74–77ºF)—but the summer temperature is a balmy 26–27ºC (~79–81ºF). The visibility is less than ideal, which is a pity since the dramatic rock formations here make for an impressive underwater seascape.
Bangkok - it's a fascinating city, but no local diving. Diving means a flight to Phuket or crazy bus trips to Pattaya or Koh Chang. It's also chaotic and traffic is a major issue. The airports can be a real hassle too.
Singapore - again diving means flying, but the city is very livable. Housing is expensive on a per square meter basis, so flats are very compact. Alcohol is taxed to near Scandinavian levels
, but all other prices are reasonable to cheap by European or North American standards. I have a very good friend who works throughout SEA and he likes it all, but chooses to live in Singapore because of the low stress level. Despite the bar tabs. OTOH, the heat and humidity are onerous and it all feels a bit sanitized and paternalistic, but that's probably a plus with a younger child.
Europe - I'd give great consideration to Europe if you can live with temperate water diving. Your son is an EU citizen who has never lived in the EU, right? Maybe give him a chance to live there full time it before he needs to make college choices
How about Trieste for a great city in a great location? As I said, I don't know about the diving there, but when I search for dive shops in the area on google maps, there are a bunch there and in immediately adjacent Slovenia.