This sounds like an involved situation. It is a bad idea to just ignore the child support payments, as the amount due will likely continue to accrue. There will also be some extra work involved just in figuring out exactly what the situation is in all 3 states, and then determining the best way to proceed.
I would urge your husband to meet with a lawyer to start trying to unravel the situation before things get worse. Child support enforcement options are serious and range from attaching pay and assets, suspended licenses, and even possibly jail. Also, sounds like there are some custodial issues to address as well. Thus, I would certainly not ignore this situation! Best bet is to meet with a family law lawyer.