Getting a paternity test for an adult child?

LOL - the son feels he deserves 18 years' worth of child support retroactively and is SURPRISED when the man decided to stop cooperating??? The logistics of getting back dates child support is this: YOU CAN'T. It can be made retroactive to the date of filing for support, no longer. If you wanted to develop a relationship with your father, then you just blew your chance to do that. He might have been willing to do something nice for you out of the goodness of his heart - but now you'll never know.