Can look shoppender nu help ?
The normal way to do this is to just keep working on improving your relationship with him. Once you have three hearts with him, have done the werewolf quests, and have visited him in his house at night you can see Bernard naked and do things with him.
On the way there, there is a point where you have to make a quick decision between Bernard and Logan. If you select Logan (Or refuse to give Bernard the cure after selecting him), Bernard will show up as an enemy in the woods which is a second place you can see him naked, although he'll not be human at the time.
Finally, if you give Bernard a love potion you can see him naked, but he really doesn't like this and thus it may have negative consequences in the future.