I am the physical custodial parent of my son but 5 years ago I signed full custody to his father but I've always had the physical child with me.

Yes, you can file for child support, but if there is current order saying the father has custody, that order need to be changed. You could either give notice to the attorney general of a change in the residence of the child and let them know you have custody or you could hire an attorney who would file a modification for you. If you notify the attorney general, there's not way to know how long they will take to file a modification. If you hire your own attorney, you know the case will move forward at a reasonable time.