Originally Posted by
Nightbus to Dalson
He's 13 but not a member of the exec club (yet)
He has a while to wait

Instead of adding him to your household account, you can instead add him to your friends and family list. This will allow you to book for him using your Avios.
Instead of selecting Household Account from the dropdown, choose My Friends and Family. Add your nephew in there and he should be visible after you log out, then back in. That was the advice given to me.