Look for a good father's lawyer in your area. File divorce by you so all court dates will be in your county since she just moved. A good lawyer may be able to force mom to return son back to your area (at least until court is over) BUT you need to act fast. The more time you "allow" her to be there, the more the courts think you okay-ed the move.
If she stays in the new area for about 6 months, they may allow her to file divorce by her so you need to file first by you.