BD787,
If your son is 19, he will presumably attend University. He can apply for a Student Pass which is valid for the duration of the course. At the end of the course, has to return to his country of origin, and re-apply for a tourist visa/social visit pass/Employment Pass/Work Permit. Therefore, not liable for NS.
I cannot comment on health issues vis-a-vis NS.
PR vs Expat: I have not heard of too many people switching to local terms from expat terms. In fact, none at all with SIA. Many expats stay on expat terms for 10-20 years until retirement. The loss of expat perks is substantial. The gain in local perks is relatively inconsequential, monetarily. A foreigner can purchase property just as easily as a local. Getting a loan is not that much more difficult. There are other advantages of being on local terms as listed earlier, of course. At the end of the day, your decision.
MC is right about the bank guarantee.