Dragon Ball Z doesn't exactly follow real world physics. A human can do a few pushups a day and then suddenly blow up planets.
If Batman existed in this world, how powerful do you think he'd get?
After his parents died, I think he might go out and seek training from somebody like Master Roshi. Then he can follow that till he gets strong enough to move on, then who knows maybe he'll climb Korin tower and get trained by Korin, and maybe even Kami eventually.
How powerful do you think he'd be?