Great question! Special Thank YOU to Eleanor...
You aren't registering it in "your" name, just your amazon account user id. You can still name it "John's Kindle or whatever you want.
This doesn't magically put all 400 of your books on his Kindle. A "sync" only updates things on that particular Kindle. He can pick and choose the books he wants by pressing the Menu button, then choosing "Archives." Then he can page through the archives on your amazon account and choose the books he wants.
Another way you can send him particular books is to go to the manage my kindle part of your amazon account on your computer. You will see a list of all your books there. Next to each one is an actions button. Click it and one of the choices is to send the book to... click that and if you have multiple kindle devices, your husband's kindle will be one of the choices. If the Internet is turned on on his Kindle, the book will download immediately. If not, it will the next time he turns it on and chooses "sync."
Additional Note: As long as a Kindle is registered to a certain Amazon account, any purchases of Kindle books from the Kindle store made on that Kindle will be charged to the owner of that account. In families, this should be easy to track and keep up with. It is a great way to pool your resources and build a very nice ebook collection you all can enjoy.