Mr Allen J said
He would need superhuman body-control, and a durability to pull that off.
His current power lets him slow down his perception of time. His developed power is effectively scaling durability, which inadvertently gives him superspeed. The body control isn't an issue, as if he can slow things down, he can do things as precisely as he pleases.
Anyway lets explain what I'm saying. Before his power develops, if he tries to move his body while his perception of time is at double speed, he will find that it causes quite a strain on his muscles. If he tries to move his body while his perception of time is around 1000x faster, he will find that he can't without turning his muscles to mush. Of course, this doesn't happen because if he tries, his brain dampens how much he can stress them, and if he actively tries to anyway, he'll get incredibly sharp pains.
After his power develops, he will find that when he tries to, his body can fill with an energy that makes him invulnerable. Not durable: invulnerable. At least, to and self inflicted damage... If he wants to move at double speed, his body will compensate for it, and it won't take too much energy to do so. If he wants to move at 1000 times speed, his body will compensate for it, and it will take a
huge amount of energy. I mean, heck: how much energy does it take to make a potato move 1000 times faster?
And how much would that hurt to get hit by? Before I get told that this is OP, how much energy would it take to keep that potato from exploding into a million pieces when it makes impact? A whole lot more!
So, if he threw a punch at double speed, it wouldn't take much energy to keep his fist together. If it hit something on the other-hand, it would take a whole lot more to keep the fist together when it impacted. And the possibility of him running out of energy to protect himself from an attack is very very real. Ashing, as stated, is also the inevitable consequence of over-use.
But this is all his ultimate potential. He can't do any of this yet.