How about a macro lens adapter for a smartphone, assuming he photgraphs his coins and has a smartphone.
I will echo some of the ideas already posted in this thread. A. Speak to his coin dealer & possibly discover the one coin he has long wanted. Make certain that there is a refund/exchange policy with the dealer after Christmas. B. Consider signing him up for the ANA membership which could include the magazine "The Numismatist" delivered monthly. If he is already an ANA member, consider buying the study at home diploma course from the ANA. If he is a good student, then he could achieve the diploma (hopefully with high honors). When he has an ANA diploma on the wall I guaranty he will smile every time he sees it for years to come. C. Books are a great idea IMO. If he doesn't already have Breen' complete encyclopedia, then consider getting him one. (I'm assuming he is interested in US coins). If I knew more about his collecting interests, then I could suggest more books. Edit to add: If he is a member of this forum, then he may not be too surprised on Christmas morning. I see now that he is interested in toned coins & error coins. If he does not yet have the Error Coin Encyclopedia, then you could possibly get a signed copy from Fred Weinberger (http://www.fredweinberg.com/). It is only about $40 but Fred has many good error coins from which to choose. Perhaps Fred also has gift certificates available.
You could check out Brian's Variety Coins or Fred Weinberg's site as he specializes in very nice error coins. A book as suggested is great. How about a book on Overton Half dollar varieties?
Honestly OP, we are all chasing the Great White Whale, or we have or we will. If you want to get your husband something really different but very much about coin collecting, try a reproduction painting, print, etc of Captain Ahab.