Restored node - missing channel balance

Hi, I restored my son’s node onto a new pi after hardware issues. On-chain funds were successfully, and painlessly, retrieved but I don’t think I had it set up for auto-backup of his channels.

Fortunately it was only one channel - to my node - which I force closed. I got the sats from my side of the channel and expected him to get his, since the address would be derived from his seed words, but no.

Is there a way to retrieve the funds here? It’s only about $30 so no biggie, but it could be instructive.