I'll share my similar story-
We have two heifers in a horse barn, rougly 102 x 75 ft., with 125 ft. x 50 ft. being a sand walking arena. Anyway- the barn is lined w/ horse stalls on either side except for about 35 ft on one end w/ open space on one side and an area that had nothing on the other that we converted to a heifer pen w/ the help of some gates. The end horse stall wasn't occupied so we put our chute w/ the blower combs sheen clippers etc., so everyday we walk them down there to blow out all the dust and put in the sheen. They are halter broke- we can walk up and easily catch them, tie them, pet them and what not, but aren't quite broke to walk. I know, kinda sad, but it's getting better. To walk them down we have to have someone behind them tailing them, and we eventually have been getting better. On the way back on the other hand- It's like there shot out of a cannon, sthey can't wait to get back. I always walk backwards (Like with a dairy heifer) and stand right in front of their line of sight and push back on the halter whenever they try to run. It maybe doesn't help that they get fed immediatly after this, but what I do is stand immediatly outside the pen gate once we get down there and scratch with the stick- at first they were fidgety and were sliding back and forth but they hate the showstick on there feet so they stopped that. After I scratch for a while I walk in the pen and do the same for the same amount of time. They've gotte alot better at not running now. My suggestion for you- Walk backwards to stop him and once in the pen work with him for some time so he knows that he realizes the pen isn't a play pen. If he isn't free in the pen, he won;t be as ancious to get back there. Let me know if this works.
Ryan