Everyone can check in online apart from DP?!

[Titsalinabumsquash]

We're flying tomorrow with VA and me and the children can check in online but when DP tries it says they can't check him in online because of government regulations?! I'm now panicking.
Has anyone had this before? DP has flown to the USA (and many other places) a few times and has never encountered this.
We all have ESTAs etc,

Check his passport info matches the passport info the airline has. When I went to NY in October I had accidentally put the wrong date on my original info. I had no problem checking in at airport after I corrected info.

Does he have a biometric passport? There was an issue with this a while ago, as it's required for US now. Pretty much everyone has them now so he probably does.

Yup, we renewed ours a few months ago before the flights were booked. It's a wish charity holiday so they booked it all for us. We sent scans of all the passports and everything to them when they booked.