Your husband needs to hire an attorney who is licensed in the state where that case is pending as quickly as possible. You shouldn't try doing the response, because your husband's rights may be jeopardized, and unless you are an attorney you would be practicing law without a license. Based on the name of the petition you stated, I am not familiar with any such form in Georgia, though it may be something specific to FL law. Also, normally a "supplemental" document is likely an amendment or addition to a previously filed document, not an initial document. Therefore, it is unclear whether that "Supplemental Petition" is initiating the case you are asking about, or is an amendment to a previously filed one. Good luck.
From the wording you are using, this does not sound like a Georgia modification action. Therefore, assuming it is filed in Florida, you could re-post your question to a Florida attorney on this site to find out more accurate information. Georgia does not have standard forms and I do not know if Florida does. However, I think you are going to find that he needs an attorney, and doing this wrong could find him subject to modification of his rights that he may not be happy with.
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To be completely honest, I'm afraid you are WAY over your head trying to do this unless you are an attorney trying to get some suggestions from fellow attorneys. There are so many issues that come up just from your question, it would take pages to address them all. The first question is whether the Florida courts have personal jurisdiction over your husband to even entertain a petition like this. Your husband needs to contact an attorney in Florida to discuss the case and determine whether Florida has jurisdiction. Then allow that attorney to do what attorneys are trained to do which is to respond in a manner that most protects your husband's interests. Please take no offense, but if you try to do this for him, it WILL turn out very bad.