Well, he was earning 1500 per month and paying tax/ni for many years.
So he has contributed in the system, same as any other worker/parent.
If no one knows where he is, then he won't be working or claiming a NI based benefit, so won't be eligible for CB.
From gov.uk
You or your partner must pay UK National Insurance (if you’re employed or self-employed) or get one of these benefits:
www.gov.uk/child-benefit-move-to-uk
Sounds like a typical DM story that is missing a few facts, to fit their narrative.