Make like a "game" get him those halloween temporary tattoos or rubber cuts and bleeding arms and faces I guarantee in a week He would be bored to see that

Try using social stories to communicate with him to see if you can learn more about the reason for him cutting himself. Role play should help as well.

How is he cutting himself? Remove sharp objects and cut nails. If that doesn't work pour alcohol to sterilize the wounds. The pain should be a deterrent. My daughter calls any wound blood and wants us to mend her immediately and she'll say if she sees blood anywhere.

can it been fake blood (from Halloween)? my son likes scary Halloween such & acting out things & the fake blood is something he enjoys at Halloween (but i'm sure you can get it on amazon anytime)
if he likes to prick himself to she his own blood, my thought would be to show him that you do that at the doctors. talk with his doctors office & see if you can come in & get there assistance with pricking his blood at the doctors & not at home.....I hope that gets your mind rolling in the right direction