Only PRs (permanent residents) and citizens are required to perform national service for 2 years. Foreigners, including those holding student pass, work permit, dependant pass etc. are NOT required to do so.
For the other things, I am not so familiar. I am sure that the language standard of singaporeans are much higher than that of Hong Kong students. Some of my friends said that they are also talking in English in their dreams. Ha ha ...... But I also think that children are easily used to new environment. It is more difficult for us (adults) to get used to new environment.
hi emmy,
welcome to singapore, on top of what dorislin said, u may want to visit MOE website? http://www.moe.gov.sg/index.htm
they've detailed info even from pre-schooler and childcare info as well.
regarding to the language barrier, i don't think u should worry about it, take my gal as an example, i speak english to her(almost 4yrs) and her father speaks to her in mandarin, her mandarin is always worse than english and since i don't speak mandarin and i was a bit worried so i sent her to chinese enrichement class for just one month, she can sing a mandarin song and even conversate to her grandparents in mandarin fluently and since my husband and i speak cantonese and she picks up cantonese from us without teaching her at all, so i don't really think u should worry about it, and we start primary school at 7yrs, if u come over this year, sure yr son has plenty of time to adept well in school here then no extra help from the school i think.