You said that you want to work on his steering? He's not the one steering, you (and your tack) are. Make sure your driving reins are through the side rings, not the top rings. Otherwise, we'll assume the tack is staying out of the way and address what might be going on here.
He circles 50% of the time because, 50% of the time, that's what he thinks he's supposed to do. Why does he think that 50% of the time? Because 50% of the time, that's the cue you (or the tack) is giving him. And maybe, 50% of the time, he got to stop after circling, and stopping (pressure release) is a reward.
In ground driving, a horse will make a sharp circle when he receives a strong rein aid from one side, and little/no rein aid from the other side. Strong rein left (with no right rein) = sharp turn left. Strong rein right (with no left rein) = sharp turn right. That's what he is supposed to do, particularly if he has very little 'forward' momentum. Imagine if you were dancing with someone and all of a sudden they let go of your right arm and put all of their effort into pulling your left arm. You're going to comply by 'giving' to the left, right? So regarding contact with the bit through the reins -- steady contact with subtle left or right pressure differences is far more pleasant.
Okay, so, let's imagine that instead of 100% left rein contact, 0% right rein contact, you have 60% left rein contact, and 40% right rein contact? That's only a slight turn to the left, so what does your horse do? Stop? Try to circle and get confused by the pressure on both reins? If so, he's forgetting or doesn't know a very basic concept: "Forward, accepting gentle contact with the bit."
(One way to develop "forward, accepting gentle contact with the bit" is to lunge with sidereins. If you start your day with this, and then try steering/turning, you're very likely to have more success.)
And finally, remember that you dictate when a 'turn' is complete, otherwise he's supposed to keep going, and will continue in a circle. So if you have 60% left and 40% right, and you're making that shallow left turn, simply adjust back to 50-50 for straightness, and then gentle pressure on both reins to have him halt. Then drop the reins, give him a pat and tell him he's a good boy.
You can also halt him from taking too sharp of a turn by putting your lunge whip on the side he's turning towards, effectively 'blocking' him. This doesn't work for every horse and can cause a real hassle.
No diet, no hoof. No hoof, no horse. No horse is not an option!