How do I overcome round pen aggression?

I just bought a new two year old gelding. He is extremely polite when I am leading him and he is very calm and mild mannered... Until I put him in the round pen. We have a very small round pen at the place I board at and he is fine in larger arenas and larger round pens. When I went out to look at him before buying him I round penned him in a large round pen and he was fine. When I put him in my round pen and started working with him he turned his towards me and refused to move. When I asked him to move more aggressively he started to buck, at me. I have him in a paddock with three other horses and he is on the bottom of the pack but he will kick at the other horses when they corner him or when they wont stop pushing him around. He also has never been lunged and when I tried he obviously had no idea what I was asking. Finally, he was imprint trained at birth and not raised with other foals/horses. Any suggestions?