Non Smoker, it is NOT possible to remain on dependent's pass after your parents become a PR. Taking the PR they will have to cancel their employment pass, so what will the "dependent" depends on??
The choice is to apply for Student's pass and that's it. Mind you, if you graduate, and then looking for job here, it is much harder without a PR-ship. If the guy wants to work outside singapore, then student pass it is. If he wants to work here..... well.
I don't know whether if he ever applies for PR in the future, the govt will consider the fact that he doesn't want to be a PR in the first place, and whether they would ultimately guess that the didn't want to be a PR earlier to avoid NS.
As others say, convenience comes at a price, so either get it and do the duty, or shun the convenience once and for all. There are alternatives.