Yes I agree with MS if the age of the boy is good, International School
application is a different visa status. Though wait until your wife gets LTVSP first and your son is rejected, after she gets the LTVSP. The son will probably always be rejected if the mother doesn't have LTVSP.
Also your wife should not return the money paid for the child maintenance, put the money away for the childs schooling and show the ex father that you are doing just that, you will give the father a sense of appreciation and i'm sure he will still feel apart of the bonding too his son.
This is very important to take other peoples feelings into consideration, so let the son have his own bank account so the alimony can be seen to grow for his future education, his father will feel more relaxed and willing.
The mother however will lose the amount of alimony paid to keep her, when she remarries and child support will continue, If the father has any financial difficulty, explain to him that he doesn't have to pay if he cannot afford it, just pay what he can into the boys education account when he can. ensure that he gets some kind of signature for the amounts paid so, that he doesn't feel that he's going to be ripped off. Also ensure he gets permission from you for any amounts not paid. This will clear any legal claim against him for failure of payment. Work together for the sake of the son and the father, his son will appreciat it has he grows up.
The worst thing for any male is to pay alimony, to the mother, only for the money to be spent on herself and not the children, think about it carefully as this happens more than you would ever realise, and the child ends up with nothing, education is everything for the childs security and welfare when growing up. Singapore laws are very different to Malaysia and access to the child will be under Sg once they become resident, however give the father a letter, allowing him access to the child anytime, he feel more accepted. Though this isn't a guarantee Singapore will allow him in to visit the child.
My point is to work with him rather than against him for the benefit of good relations. One must accept that first husbands and families remain a part of your life now, and entitled to see their grandchild when possible
, work together and you will all be fine.
I think if you offer the ex father the choice of becoming involved in his sons future is a very important step for him and the son, especially when the child is older.....I'm quite positive all will work out well, it just takes time on immigrations side, I think there is a technical problem some where, I just haven't figured it out, it maybe a waiting period of marriage before on LTVSP, as I am sure the work permit is blocking any dependant, so I wouldn't bother applying for the son until your wife has the LTVSP include the sons details on her application if this is a requirement. Good luck