Who found his car? This sounds much better to my ears as a native English speaker.

This is hard, actually. In almost all cases, I think, you would say "who found his car?" and not "who has found his car?" I've thought and thought about it, and I'm afraid I don't know why. BUT there is one situation where you would use it: - Have you heard? Jim has lost his car! I've been searching for hours. - Oh, it's all right, someone has found it. - Really? Someone has found it? - Yes! - Brilliant! Who has found it? - A boy. He found it an hour ago. Even in that case, "who found it" would be more natural. "Who has found it" implies that the finding is still "current" in some way. The speaker certainly doesn't know it happened an hour before. That's my feeling anyway. I hope it's not totally confusing...

"A boy who found his car an hour ago" is correct. When we are thinking of a specific time in the past, we must use the simple past, not the present perfect.

Yes, they're correct. There is just a difference in tense (which leads to different meanings).

Yes, both are correct. Although, it is variable in certain cases. For example, if you are asking question to a group, then you might use - who has found his car? 2. Or to ask it to a single person, you might use- you found his car ?? Just tried to clear out.