Attempting to fix this...
What is the name of your state (only U.S. law)? Florida My husband needs to file for child support in florida. My husband and his ex wife have a divorce decree from 2004. They had joint custody so no one paid child support. That changed in 2016 where he now has full custodial custody and she does not pay anything for their support. How does my husband go about filing? Do he just go down to the courthouse and get the forms or do he need to go back to the court with the divorce decree? and how much would this all cost...thank you for your patience, as this is really my husband's legal issue and not mine.
1) Hopefully your husband and his ex actually have a court order that reflects the current parenting plan. If the parenting agreement has changed, but it was not filed with the court, that would be the first step.
2) Did your husband petition to modify custody when the parenting plan changed? If not, then why not?
Again, you are a legal stranger. While I can understand your desire to help your husband, legally he's the one that has to do this. You can nag him, but ultimately he's the one that has to stand up and deal with this.