You might want to post this on the CA Avvo site for attorneys from CA to weigh in on this topic. After 21 years practicing in CO, I have heard plenty of estoppel arguments, but never a "parentage by estoppel" argument. So, that is a new concept to me. Normally, the biological father has an obligation of support and a right to parenting time. Because of the mother's actions, you should be entitled to a HUGE amount of makeup parenting time with your son. You would definitely be well served in obtaining an attorney to fight for your rights as the son's father. The attorney that you hire will be able to tell you whether any appeal is possible, under what theories, and whether the brief time period for an appeal has already passed. Good luck
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I doubt the estoppel argument will work but you may be able to avoid some of the previous support payments under a laches argument. However, it probably won't affect future support. Get an attorney from the state that entered the orders.