Threatening to deny visitation -I'm out of state

The issue at hand is that my mother and I are planning a visit from August 19-August 23 in NC. My ex husbands spouse has informed me that they will not let me see my son until I pay them “back child support”.
Per the request of the Concord Police department, I am sending the them a copy of the WA order, the sheriff’s department is no help.

Some background:
My ex lives in Concord, I live in WA. We have a WA court order that spells out our parenting plan. This past year we mutually decided to let my 11 yr old try living with his father for the school year (2003-2004).
Our verbal agreement was no child support, but I had to buy plane tickets. Phone calls would be the same as in WA, twice a week at scheduled times. My son has been doing great in school, however, his dad and step-mom refuse to assist my son in making phone calls, (reminders, etc). They are also demanding I pay child support, it keeps going up (from $50 to $600 a month). They keep telling me they are going to file for sole custody, but never do. I would welcome an NC order t sort out the support, visitations, etc.

Any ideas on how to make sure I see my son? I was able to complete all the original paperwork in WA via online forms. From what I’ve read, NC is a nightmare to try and do yourself.I don’t have funds for legal representation in NC.

Thank you