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Is there an actual custody case in either state? If not, you are not required to return your child to her at the end of the summer. However, any new custody case would have to be filed in Florida. It would be a good idea to file in Florida. If you simply refused to return him, your ex could file in Florida and ask for an order returning your child to Florida. The chances of this happening are reduced if you filed in Florida first. It is also reduced if you have already had your son with you for more than a month.
If there is an order from either state, you must obey that order but you can request modification of that order at any time. You can request it on an emergency basis if there is an imminent danger to your son if he is returned to his mother's custody.
If you can prove that he has actually been living with his grandmother, not his mother, this would help your case because parents have more rights than grandparents. The Court will not allow a grandparent to have actual custody before a parent without showing that it would be detrimental to a child to be with a parent. Since you are a parent (I am assuming paternity has been legally established) you have priority over grandparents.
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