Your court order is your guide. Reasonable means exactly that. A nine-year old is not developmentally mature enough to spend an entire summer away from the primary psychologically bonded parent under normal circumstances. This age child might be able to do two or three separate one week periods with Dad, but the child's maturity, needs, and relationship with the father need to be considerations. If Dad was not concerned about what is best for your son, it seems he would be willing to come visit first in your state, and get to know his son better, before taking him out of state. I would recommend shorter visits at first until you see how it goes, and if it goes well, and father cooperates with allowing telephone contact with you during the visit then you can always agree to increase the visit the next time. Check with a local family law attorney to see what your local courts expect of you.
I don't know what state you and your son live in, and I can only practice in Washington State. However, I would let the father take you to court (you can't stop him anyway) and then defend the case he brings.
A whole summer is a very long time for a child of 8 or 9. That would also mean that you get NO SUMMER VISITATION - how fair is that?
If you and Dad live in different states, he should get a few but long visitation periods, like Winter Break (1 of the 2 weeks off from School at Christmas time), Spring Break, and about 2 weeks during the summer. He should be forced to pick the summer dates by April 1st of each year, or else you can set your schedule and he will get no priority for dates - but still he should get some summer visitation.
Hope that helps. Good luck.
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