Fetishes and Paraphillias are intensely personal in their nature and I don't think shaming is the way to deal with it. having been in a similar place to your bf, I know it just made me obsess more.
It's difficult to Gage just how to reply with out knowing what it is he is fantasizing over, but I am going to second what phreak said and add that it's better he learns to deal than deny.
I really do wish you both all the best. But be honest about if this is something you can live with if he is unable to address just now.