Different families have different habits

The traditional ones would prefer to be called Mom and Dad; More open ones don't care if you call by their first names.

My point is: listen to your bf! If he think it is appropriate and works in his family, DO SO! He knows his family better than anybody else! You call them the wrong way, which you get based on other opinions from other family's habit, you feel weird and your bf and future in-laws feel weird as well. Not a big deal what to call, but everyone needs to feel comfortable. Otherwise why bother go see them?