If you owe more than $2,500 in child support your passport can be restricted or revoked. I would imagine that if this happens he would not be able to get back to the US and would need to make his payments to get his passport back (it actually says he would have to apply for a new one)
Perhaps that is angle you could work though I am not sure the US embassy will force him to turn over his passport.
OP you really need to get official legal help. I can't recall 100% but I think you might have been the victim of some sort of abuse, if this is the case then I am sure there are resources out there that can help you for free or no cost.
So I guess part of the issue is this, did they tell you they can't collect, as in they can't enforce the terms of the CS agreement, or rather that they can't even set up an agreement? I would think as long as there is an agreement, if he defaults he is subject to the same punishments as someone working in the US regarding liens, passport, account freezes, etc
http://info.legalzoom.com/passport-held-due-nonpayment-child-support-24277.html