KJ also took to his Instagram Story to share a gruesome close-up of the injury and explained what had happened.
He said: "I split my head open yesterday.
"I don't know if you guys can see that, but I have two stitches in there. Metal stitches."
The on-set accident came just a week after the "Riverdale" star was left in pain when he got a piece of metal stuck in his eye.
The actor shared a two-minute video on Instagram of the agonising removal process.
He captioned the video: "a shard of metal got stuck in my eyeball (sic)"
In the clip, he told the camera: "I'm not trying to do that. I'm not trying to do this right now. I have to work tomorrow! It's actually like punctured my eye!"
Once the piece was out, his pal Jason Schneidman, who was assisting, said: "You got the piece out, bro. That's the piece that was f****** in your eye."
KJ later reassured fans that he planned to seek medical advice but was feeling "fine".
He said: "Quick update here, guys. The eye is good. I'm seeing a doctor tomorrow. My appointment's at eight o'clock.
"I just want to say thanks for all the support. I appreciate it a lot.
"My eye's fine. But there could be some more metal shards in there, rusting away in my eyeball."
- BANG Showbiz