yes.. only 2nd generation PR must do NS...
Apparently that's the official policy. I dunno if there was a change of policy towards this, cuz I distinctly recall serving with 2 Indonesian Chinese/local PR's back in the mid '80s. Even served as a character witness for one of them later on in NUS.
Back then, the way I understood it was: if they didn't call the male PR up to register for enlistment when he was age 16+, then - heng, ah - he's likely to have successfully "siam" NS.
Ronald Susilo, an ex-PR who went through the local education system, got his citizenship papers at age 23 &, up till now, hasn't been conscripted at all....