This is a bit harsh, but with further searching of this board on your part you will understand why.....
If your son get PR, yes he will be obligated to do NS. If you get him PR and then rescind it prior to his NS registration date (16.5 years old) you will have to have him leave the country forever as he will be considered a NS dodger. Additionally, the odds of the parents losing their re-entry permit are also very good because they are probably the ones who instigated the whole thing. However, if you don't get him PR but keep him on either a ltvp or Student Pass (you didn't indicate your son's age) he will not be able to get into a local school so you will be saddled with sending him to expensive Int'l schools.
It's NOT YOUR DECISION except up until the age of 13. After his 13th birthday there is no hope as if you get him out after that, you will lose your PR as well. As well you should. If you want a clearer picture than what I'm willing to spend on someone who only wants to take from this country without any quid pro quo, I suggest you do some searching in the "Strictly Speaking" Sub-forum under the General Forum starting with this thread and many, many more just to see what you are getting yourself into....
http://forum.singaporeexpats.com/viewtopic.php?t=61423
My advice? Stay in Canada. Your type have already given the Indian diaspora a bad name in Singapore for this kind of stuff.