If you do not have a current custody order then you will need to file for custody. Since you had a previous agreement, you need to go pick up your son. If you have a signed agreement, take it with you. You may consider going by the police station to ask them to go with you to pick up your son, if the agreement has custody listed in it. Extra visitations can be arranged but are not generally forced on the custodial parent.
If you have custody agreement then your ex will have to file for custody and due to the previous posts about the contact, his actions, and your current arrangement, it’s unlikely that he would win.
The courts do not generally split up siblings and will not look kindly on the fact that your ex has allowed you to have primary custody since the separation and is now using your son as leverage. This could be considered kidnapping, depending on what your agreement states and given that your ex is asking for “ransom”.
You need to consult an attorney where you live now if you have been there long enough to be considered a resident. If not, then you need to see counsel in NC. File for equitable distribution. You need to make a list of the marital assets your ex took and what you kept. You need to make out a schedule of visitation that will work with your ex’s military duty. You are entitled to 1/2 the marital assets. If your ex kept 90% of the marital assets, then he owes you 1/2 the value.
You are allowed to rent a home from a friend. Make sure that you have record of the rent paid, to protect all of you. It is not illegal and your ex can do nothing about it.
I do not know if you are or can file in NC, but if so, there is little that he can do. Regardless of whether or not he signs the papers the divorce will go through. Again, that’s if you are filing in NC. You will need to check the state you live in now. Please consult an attorney.