It sounds like he doesn't like your bit. Every time you put pressure on the reins he tosses his head? That is evading the bit. I would probably put him in a a double jointed snaffle like a french link.
It will be difficult to get a former barrel horse to be wp slow. You are in for a long ride! But it sounds like your off to a good start. You need to be patient because he doesn't understand SLOW. Let him be a bit fast for a while, then ask for slow, reward anything you get, even if it's only 3 strides!
I would suggest jogging or loping for a few strides then stopping, backing and taking off again and repeating this cycle until he slows down a bit, but this may backfire on him. It might make him hotter because of his barrel racing past. But it may not. It'd be worth a try at least. If not, circles, circles, circles! It might sound mind-numbingly boring but it does help. Take him into a circle and don't let him go back to the rail until he has slowed down. If he speeds up again when he gets back to the rail, start over.
I know how difficult it is to turn a non-wp horse into one. I'm still working on mine! Lol We only do wp for fun because he just wasn't made for it. He's a hunt seat horse. Good luck!