I have been been paying $550 month and health care costs to the mother of my 11 year old son since he was born and we never had paternity established or any court order to pay. This was simply a verbal agreement to pay that amount. I have now been granted guardianship over my son and he is nearing the age that he can state a preference and if he goes back to live with his mother, the state will determine that I owe $1150 per month in child support. Can they also go back and collect support from those years that I only paid $550? If so, how far back can they go? We did not have paternity established for those years but do now.

There are a few things happening here.

1) Do you have a record of the payments you did make so far?

2) What state are you in or the state the child support order will be through ?

3) The child never chooses which parent to live with. Its always the courts decision. Stating a preference does not make it so. The courts reserve the right of "best interest of the child".

I do have a record of all payments made and I live in Oklahoma

I have been been paying $550 month and health care costs to the mother of my 11 year old son since he was born and we never had paternity established or any court order to pay. this was simply a verbal agreement to pay that amount. I have now been granted guardianship over my son and he is nearing the age that he can state a preference and if he goes back to live with his mother, the state will determine that I owe $1150 per month in child support. Can they also go back and collect support from those years that I only paid $550? If so, how far back can they go? We did not have paternity established for those years but do now.

Who told you that you'll owe $1,150 per month in child support? What I'm reading here:
OSCN Found Document:Computation of Child Support Obligation - Percentage of Combined Gross Income of Both Parents - Computation for Each Parent Required - Transportation Expenses - Support Order - Summary Form - Social Security Numbers Required (http://www.oscn.net/applications/oscn/DeliverDocument.asp?CiteID=452146)
Is that child support is based on both parents' income, not solely your income.

Assuming that your case is in Oklahoma, you can go here:
OKDHS - Child Support Calculator (http://www.okdhs.org/onlineservices/cscalc/default.htm)
To get a rough estimate of who would owe what in child support based upon who the child resides with primarily.

And the courts may not even accept the money you paid as child support, since there was no order for child support though the courts. When we say a record, do you have copies of cancelled checks, or some other proof that they got the money, not just a list of when you paid.

I have the cancelled checks for every month I have paid support. The figure of $1150 did come from the OKDHS website.