You get his first heart from the hunting trip, when you sleep on the same bed, his second from a repeatable mini date you can have at Hayden's Onsen, the third you get after the werewolf thing and the fourth you get after doing an improvement to bareshade and watching the fireworks show you can prepare there
After finding the cabin in the woods that belonged to Bernard's friend, a dialogue will be unlocked with Bernard about him going hunting every friday.
(also a little side note Bernard's hearts aren't tied to the werewolf event so you can not choose him at a later thing that happens in Bareshade without consequences)