Not sure I see a solution other than alternating years or two hours apiece.
As for your ex - not to sound overly suspicious, but please run a credit report on yourself and your son. Probably your ex is just running up a ton of debt in his own name, but sociopaths like him HAVE been known to get credit cards in their ex's or children's names and run them up. I'd check just to be thorough. That sounds like an awful lot of money he's been spending.
