I think you might want to check out some international-based schools then. The expats here generally send their kids to them. If your son is Singaporean, I think there is a clause stating that he is not allowed to attend International School
(if your husband is not Singaporean, you can apply to the MoE for an exemption - never heard of anyone being rejected).
If you are both Singaporean, then the best option for you will be Eton House - it's an international-based pre-school and primary school which is open to Singaporeans and expat kids. Tonnes of expat parents send their kids to this school because they like the fact that their kids are going to mix in with local kids too. As a mum staying in Singapore, this appeals to me too - but since my kids are still very young, it's not something I have had to plan yet.
I think Eton House charges around 12,000 per year in school fees, but it depends on the age of your child.
I hope this helps!