Should I ask about his past?
Maybe. Why are you curious?
I like him but do not know much about him.
It's perfectly okay to be curious and ask about a person's past as long as they are comfortable with sharing that information with you. If they don't however, and prefer to keep that information private, it would be rude to ask since it makes an uncomfortable situation. It can be hard to tell which situation is which without more context, but as long as you just ask the person once and not press them and bring up the subject multiple times, then it would be socially acceptable.
I just wanna know his past preference.
Then that is okay. It is good to want to know their preference so that you don't do something to upset him.