What size area are you keeping him in? Its recommended to keep an unhandled/ungentled horse in a smaller area initially before letting them out into a bigger area. When I had a 5 year old unhandled haflinger stallion come here we started him out in a stall for a few days and he quickly came around where we could halter him and pet him all over. Then we moved him into the 50X50 corral before finally letting him out into the pasture.
I normally would, but this guy is scared of people and has a strong flight response, plus he's 18hh and I would guess 1600-1800#. We have great stalls, very sturdy, but I don't know if they could handle an animal this size trying to climb the walls. I've come to the conclusion that my best option is the round pen, where he can learn that standing still is alot more pleasant than running circles and where he has the room to not feel claustraphobic. I'm 5'5" and not overly big or strong, so training him is all about the mental side of things.
He is in a very securely fenced 100' by 50' stallion pen, with daily access to a treed half acre pasture. He also has a big stall in the barn, which is always open for him to run in when he wants to. He comes in for a bit of feed when the herd does. He will follow me in and out of his stall, across the yard and into the round pen. I can pet his face while he eats his grain.