After he likes you enough and you've gone on "dates" together (hunting trips mainly, you can also go to the tavern together, but I don't know if it's before or after the hot springs) and eventually you can talk to him about the hot springs. (if you already have that option and it's not marked as new then you've spoken to him about it).
After that just show up at the hot springs in town Saturday and/or Sunday evenings after 18:00 and there will be an option to Wait for Bernard.
Don't insist on paying or he'll be offended and like you less.
You get to see him fully naked (frontal image only currently) every time you do it, which can be twice weekly.
Very, very nice view.