I had a nightmare with the CSA. They can lie and be aggressive.
It may well not be pretty, but if he's defaulting, it's his fault, and he's asked for it.
Please, please tell me he works in a salaried/waged job. if so, no problem you'll get the money eventually. If he's self-employed, be prepared for a real problem.
ExDW and I had an acrimonious split, and I left when she again told me to leave the house.
While I was sorting out things (out of the house) she found out [by breaking into a work filing cabinet] I had been having an affair.
I continued paying the mortgage and support; did the CSA calculations and paid her rather more than that. I left the house with my clothes and books and that was it.
FF past the bit where she did me in court for ancilliary relief, ending up with her paying me £6k (which £3k went on my legal fees; hers were £8k); and her being difficult with access to the point where I refused to communicate with her except in writing, which I'd let the children read and discussing Family Court for access orders...
The CSA decided they'd check up; they told her they could arrange collection (obviously much easier than the standing order from me direct into her bank account) and then promptly told me I owed £8k.
The reason it didn't end badly is I knew their reputation; sent everything to them Special Delivery, photocopies retained; recorded every call with them, and did transcripts.
Note, this was even with exDW saying I'd paid since I left.
He will end up with 20% of his income going for 2 kids. Yes, the access may be an issue for you - but surely you'd want the kids to see their dad 2 nights a week anyway?
I had to fight like hell to see my kids. The reason I got access in the end? The kids told their mother they wanted to see me; and told her that's what they'd tell the judge.
DS/DD are now teetering into adulthood. I get on with my kids really well. That's what I can never forgive my exDW for trying to break.
If he doesn't want access - well more fool him, and take the 20% and smile.