According to regulation, a service member must pay "reasonable" support even if there is no court ordered support. There is no set amount they are supposed to pay, but the regulations contain guidelines. If a service member refuses to provide support, they can be subject to UCMJ action for failure to support. The military will only take money mandatorily from someone's paycheck if there is a court order, however.
If someone is refusing to pay support, you can contact their command, which is required to investigate and counsel the servicemember on their responsibilities, and if need be initiate UCMJ action. The military much prefers that there be a court order, however, in which case it can just take the money off the person's checkbook.
Army Regulation 608-99 is the primary regulation, and can be found at:
http://www.jagcnet.army.mil/JAGCNETInternet/Homepages/AC/Legal%20Assistance%20Home%20Page.nsf/0/d3feb7e9945926a9852569100056f8f9/$FILE/AR608_99.pdf
