You are bound by the order of the court, until it is changed. That means, for the time being, the smart thing would be to send the child back to his father's.
That said, if the child is 15 and wants to stay with you now, there is a very good chance you can modify the court's order. Any time there has been a change in circumstances, the prior court order can be modified.
A 15 year old child is not given absolute authority to decide which parent he will reside with, but his wishes will be very heavily weighed. The older a child gets, the more weight the court puts on his wishes. At 15, unless the child has some other problem you neglected to mention (mental or emotional issues that would make his judgment less sound), it would be very likely that the judge would do whatever he wanted, as long as it was reasonable.
If you choose to just keep him with you, the father may file a motion to have you held in Contempt of Court for willfully refusing to obey the order of the court. The penalties range from fines to even possibly going to jail. The other issue is that it may end up backfiring on the visitation time. If you keep the child with you, the father will probably be allowed to "make up" any time he missed, even if the court grants your motion to modify.
So the correct legal process is to obey the order for now, and file a Petition to Modify as soon as possible to get custody changed.