You can't really tell what you don't know. Maybe you can work on this as it sounds like it is a recurring theme for you with his family.

You do know your childhood wasn't stellar. So start with that. Not gory details just that you weren't a very close family and that you are on your own now. And you aren't real sure where your family is and change the subject.

Always be truthful. But it is your choice as to how much you share Try to come up with some actual good memories & stick with those.

You are lucky you found a family in your bf's. I wish you all the best.