Okay, all you round pen and longeing peeps who let the horse turn in towards you....DON'T DO IT!
A horse 'facing you' is a position of dominance, NOT submission. The horse should wait for you on the circle and you should 'go to' the horse.
If you watch carefully, the people who actually know how to round pen ALWAYS turn slightly away from the horse, inviting the horse to come to them with their body. If they don't turn, the horse doesn't come.
It's the same thing when teaching a horse to lead or longe...shoulder and hip position is VERY important. Close either and the horse turns to you, open them and the horse stays where they are, unless you open way wide then you invite for a change of direction to you, or close way shut then the change of direction is away from you.
For the OP...stop allowing the horse to come to you by 'anticipating' his thought process to turn. The VERY INSTANT the horse thinks about turning in, you must ask him to go forward anyway, anyhow and prevent the turn.
At this stage a surcingle and side reins and bridle are a no-no. Put him in a longeing cavesson and work him in that until he's educated.
Don't focus so much on the whoa. It'll come. Put him in the cavesson, ask him to walk and let him walk all day long if he wants. Don't let him turn in and when he gets tired of walking...in that moment...when he wants to stop, you're going to ask him to whoa and you're going to be VERY clear with your aids. You're going to step towards his shoulder....not towards him....but parallel to him...to block that shoulder and your going to give him a half halt, or two, or three with the longeline and your going to use the proper voice aid as he comes to the halt. Do NOT say, whoa, whoa, whoa, whoa while he's still walking. Wait until he's going to whoa, and then say 'whoa'.
Do not longe him in just a circle. Do straight lines, half circles, full circles. Put out some ground poles to walk him over etc....
If your problem is that he gets going around like a fruit loop, then longe him in an enclosed area and run him into the wall. He WILL stop, just be prepared for him to turn away, or turn toward you as he looks for an escape route while he reacquires his brain. Hold him to the wall with your body. Push too hard and he'll turn away. Open too much and he'll try and run between you and the wall.
Rinse and repeat in the opposite direction. If you do a good job being consistent and being ahead of his actions, it won't take but a couple of sessions and he'll have it.