I agree with the above post.
A horse shouldn't honestly be that fidgety with a bit.
Yes, he may not like the thickness/shape, but he can deal for a little while.
I'd suggest moving it up a hole. If he's trying to spit it out, sounds like it's too loose. And, with a single joint, if it's too low in the mouth, the joint is going to more "pokey".
I'd try a french link, or an oval mouth on him.
I LOVE JP bits. Awesome shape to them, and they have basically every snaffle you could ever choose from.
"You're just as sane as I am."~Luna Lovegood.