Luckily, in SAS you should not encounter too much of "Singaporean society way of thinking". To be frank in Singapore any form of disability, learning difference, special needs and so on is treated by a majority of people (not everyone of course) without a basic understanding. People are afraid, shocked or sometimes ashamed. What is really sad, it happens not only among those people who have nothing to do with such a problem, but also among teachers, parents and other caregivers.

Of course this is huge generalization, but indeed it is so different here than in other countries (Europe, AUS, US), that it may be a significant 'culture shock'. On the other hand, it is getting better! At least right now you can see social campaigns supporting special needs education, or simply educating the public. I can see changes within the years I am here.
If your son is so called (I hate this term) highly functioning, most of the people will not notice there is something going on, otherwise you may expect staring with mouth open, or even comments... in case he will act unusually in public.
Well, on the other hand, it happens when they see neurotypical people as well

(i. e., pregnant woman, couple holding each others hands or any other 'reason'. TV works similarly to 80-90% of Singaporeans

). That's uniquely Singapore
I do not know if SAS indeed support children on the spectrum, but I do know they do have a great group of speech therapists there, and they do accept kids with learning difficulties such as speech and language delays, attention deficits of other learning differences, so I guess PDD-NOS should be also OK.