I'm just back from a week away, and had almost forgotten this since playing catch up today but thanks for pointing me to that Peter!
I knew there could be a lot to it, but from looking at the VAT Notice 700 it seems quite clear from section 14.2.2(a) that he should be declaring this when he receives the payment.
I will point him to this so he can see for himself. I'm hoping he's not going to be difficult about it but I'll see what he says, and find out for sure in a couple of months!
Thank you