As far as my knowledge goes about the Temporal Paradox nothing would happen in our Timeline, it would change his Timeline. This is Because our timeline is already pre-defined, him punching his past self would cause himself to be having a hard one about remebering where the f*ck he got that bruise from. Otherwise, if you would travel back in time it can only change things for you. And you only you will wake up in a Different world. Theoretically Somewhere outside of our universe there is a Multiverse doing exactly the same as ours. Its something about a 10th Dimension, where there are Multiple earths and Multiple Milkyways running at the same time, soooo according to this theorie, IF our Universe is "Quantum-Locked" to Different Universes time Travel would create a new Timeline different from ours and thus the one will either collapse our swap out ours and sync that Universe so it plays after the rules of our known 4th Dimesion.
1st Dimension is a Straight Line
2nd is Up Down Left and Right
3rd is Up Down Left Right Forward and Backwards
4th is Up Down Left Right Forward Backward and defined by Time
5th is Up down Left Right Forward Backward defined by time but also Intersecting with another 4th Dimensional space on the same Plane.
Dont ask me about the 6th and Higher cause that shit f*cked up.
Something about the 100th Dimension is where you can basically play god and Have multiple branches of Multiple Multiverses in Multiple Timelines with Interlocking Multiverses in the same state in a Different time. Whatever you do, he would not be able to collapse our Universe with a Simple smack to his past self
Sorry quick edit*
Our Universe is defined by "Time" as far as it goes only if you could travell faster than Light you would Travel Back in Time, but thats basically Impossible because we are like a Character in a Game. Only if we could find a bug that allows us to quote "Break Lightspeed" you would go back in time and maybe even cause you to be dead in a Matter of Milliseconds. You travelled at the speed of Light, you grew 80Years old in a Nanosecond and didn´t notice it.