You haven't specified the type of 'special needs' education your son requires. Is he currently attending a special school, or a local public school with extra classroom assistance? Have you checked with the Singapore American School as to any provision they can make for your son?
Other schools to check out are: Dover Court School in Dover Road; "Horizon School" (majority of pupils are handicapped to some degree I understand), and for more severe special needs I understand that there is a school called "Genesis" set up (I think) by an American expat; this is located near the Botanic gardens. There is/was some information about this school in the FAQ section on moving out and setting up in Singapore. Also a search engine should be able to find you more info on these schools if there is not already a link on this site.
Hope this helps!