My DIL's ex skipped the really minimal child support payments for his daughter too and my son and DIL took up the slack. Once he moved back locally of course it was all about enforcing his 'rights' for access, never mind that he worked off the books so his child support couldn't be deducted from his wages. It did him no good in the end - he hasn't seen her for years.
What I couldn't understand was how he could let another man pay to support his child. You'd think he'd have more pride.