What can I do to convince my husband to whiten his teeth?

Whitening . Whitening is a personal decision and what if he gets severe sensitivity how are you going to deal with that problem. Never force anyone treatment because if it does not work out you are the blame.
May not work..... Unfortunately, whatever the condition of your husband's teeth, you can only suggest what he does with them (the same way he has no real "say" about your hair color or style). We find that beyond suggesting, he'll only resist more if you continue to bug him about it. Some people whiten their teeth after they realize how nice others they're close to look after whitening, so be patient.
Gift it to him. Buy him the whitening as a gift, and pre-pay for it. He won't want to waste money, so maybe that will convince him to get it done. Only a suggestion, don't really know about your relationship or if this will open up further problems... But it is a good idea.