I believe how it works is that your husband received the 15 night bonus for being approved for the card, and will continue to receive a 15 night bonus each anniversary that the card continues open and in good standing.
Also, the fine print of the credit card application states that the bonus points will be credited to the account within 8 weeks after the first statement. So if you have already passed that mark, you might want to try contacting Customer Service for further assistance.
Your husband is one of the lucky ones. The agreement states that you will be credited with the bonus nights when you sign up for the card and at each anniversary provided the annual fee is paid and the account is in good standing. I on the other hand have been waiting since March for my bonus nights to be credited to my account. I have a string of emails to customer service, each one stating the nights will be credited within 6 to 8 weeks. I would consider myself lucky if I receive the bonus nights within 6 to 8 months!
You can see if you have received your 15 nights elite credits if you log onto your account at MarriottRewards.com...if you click on the My Accounts page, and look at Marriott Rewards Activity, you'll see a section in the middle of the page called "Nights Credit toward Elite Membership". There is a drop-down function, and if you click on that it will break out your nights stayed from your Bonus Nights (aka nights from visa card). They don't make it easy to find!!!