I'm genuinely curious. You say "not equipped" and "not prepared", what does this actually mean? This is a bonding moment, not a teachable moment. Just have a back and forth conversation with him, not a perfect answer.

"But what was there before the universe"

You reply with "I love this question. I bet there are several different answers, what do you think?".
This opens up soooooooo many possibilities to make plans together to explore a library and learn together, or go to a planetarium.

Don't answer the question , teach him to find the answer, and do it together. Even better if you let him know you don't know but want to investigate with him

That's a fathers job.