Actually, I think he's the defaulter, not the parent. He intelligent enough to be able to hunt down other information on prisons, etc. and he didn't know about how old the father was when he arrived here. If they are rellies, I would suppose the writer would have had to be of the same generation at the father, therefore should have had some idea. Whereas the writer, if the son, was born in Singapore, may well not have known how long his parents were in Singapore of if the father had done NS or not.
We are talking to, what appears to me, as the son who was screwed by the parents first and has decided he doesn't want to return to face the music now that he realizes that he is well and truly phucked. Speaking of which, the son would be a lot more adept at the English language and picked up on the notation between deserter and defaulter, which leads me to believe an Australian education.
I might be wrong though.