Claimed my son by accedent on my refund I am custodial parent more then 6month of year the divorce decree says dad claims being asked to amend is there way around this?

You could give the ex the difference between his refund without a claim for the child and the refund if he did claim the child.

How would one go about finding out the difference in that situation? I mean the difference in claiming and or not claiming? 

Both you and your son's Dad would have to prepare a "Test" tax return where you do not claim your son and your former spouse claims your son.  The difference between the refund amount on the "Test" returns and the returns filed would be the additional refund amount your son's Dad's would have received.