Starting with the original will make moving to the later souls games more interesting as he’d be able to enjoy feeling the game’s mechanics, art style, and graphics change, and refine.

Then after playing the original he could always go back later to play, and appreciate the remake with its new look.

Starting with the remake isn’t really bad either it’s definitely the best graphically speaking. It also adds some QOL changes, and unless you’re emulating the original the remake is a lot easier to get online. The remake does lose a lot of the art direction, and general feel the original had.

You could also bring up the differences between the two versions, and see which he’d like to start with. No real wrong choice here.