Did your husband use a Promo Code when he signed up? If so, as lensman pointed out, you probably ran up against these restrictions:
"New accounts must use a different credit card number from the referrer to qualify for rewards credit for the referrer. Customers also cannot refer themselves or their spouse/civil union partner. No exceptions will be made.
Promo codes must be unique across our entire system. Promo codes only work for totally new customers (not for existing customers adding more services or new customers trying to host domains that are already in our system)."
If you really want the domain hosted under a separate account, maybe they would reconsider if you offered to pay the “full-up” hosting charge, then again, they may not.
It would certainly be less expensive for you to just host his domain!