I doubt that you can claim child support for your son now that he his past the age of 17 or 18 years old. The point of claiming child support is that you have to be supporting a 'child' for this to ever even work out for you and now by law your son is no longer a minor in the eyes of the law at least. Yes, that stinks that you have to now support your son without any further or current help from the government, but that's just how it is. Now, I'm not sure how the law works when it comes to receiving past child support that you did not claim within the past year or two, but I doubt that the state will ever give you the money that you lost from a bit of ignorance on your part when it comes to child support.

Perhaps the laws in your state are different from the majority of the rest of the states in America. If I were you I would call someone in which can help you out with your issues on claiming your child support money from the government or you look up your local laws for child support in your area.

Child support is not a deduction for the payer, and is not taxable to the recipient.

Dependency for tax purposes is a different issue.