OK, how is there a court order for the oldest but not the youngest and how is it that you're so far away from him & then that it takes a flight to get to see them? Was there a final divorce decree? Is this a temporary order? Is your move away pretty recent?
Were there any agreements between you & him in writing about what time you were to get?
OK... with the oldest, since there's a parenting plan, he can't change it without court order or your agreement. If the plan said you were to have the child for the summer, then that's it, he has to give the kid to you. Take a copy of the parenting plan with you when you go to pick up the kids.
If there were any changes to the plan or written agreements (even in e-mails from him) that he was going to let you have them for the summer, then you can try to enforce that.
Finally, maybe the way to go abotu this is to appeal to his parenting instincts about what is best for the kids... to spend time with you since they'v enot seen you in a while. If he has it in writing that you'll return them at the appropriate time... maybe it'll be OK. Have you asked him exactly what his concern is about them spending the summer with their mother? Maybe he just thinks they shoudl go to camp with thier school buddies and maybe you can find a way to negotiate that for them to be able to participate.
if it's just that he doesn't want you to ever see them again just because you moved away, it'll look really bad for him. The courts recognize that kids need both parents, and the refusal to negotiate a reasonable solution will look really wrong.