You can try Genesis school on West Coast, it deals specifically with special needs kids, most of whom are international. Dover Court is a mainstream
International School but has a big special needs department within the school, they do an assessment to determine if they are able to assist you.
Otherwise there are the local special schools, you can get in to them without permanent residency, however as a foreigner you are placed behind the locals on the waiting lists. It is not however impossible to get in, and can be worth contacting them.
There are also organisations here for most disabilities etc and they may be able to direct you to the best places. I canonly suggest you google the particular disability for singapore and see what comes up.