The N600 route is really designed for getting US citizenship for children whose parents intend for them to live overseas. The route to citizenship for parents living overseas with their children who intend for the children to reside with them in the US is the immigrant visa route.
Under the Child Citizenship Act, if you enter the US with the children
In your legal and physical custody;
They are admitted as legal residents (i.e., they have immigrant visas);
Your intent at entry is to reside permanently in the US (as opposed to a visit); and,
The children are under 18
the children will become US citizens immediately, with no additional paperwork (although you should get US passports for them as soon as possible).
If you can do this, you will not, therefore, need a financial sponsor as that is required only for Legal Permanent Residents.
There is, of course, an issue with your son turning 18 so soon. If you can get a written job offer from your cousin (or any other job in the US), the only wash I see this working for your son would be to approach the Embassy with the job offer and the fact that your son will turn 18 in less than a year, and request (beg?) them to let you do DCF.