How can I fix dangerous spinning when mounting my horse?

Hi there. I recently adopted a 14 yr old Morgan gelding. He has had extensive training by his previous owner but he was just too spirited for her (she was 54 and never rode previously in her life). If he refused or acted spooky with her on his back, she would simply dismount and walk him home. I was made aware of all of his quirks before purchasing him and warned of his stunts. He is very intelligent and picked up a very dangerous bad habit with his previous owner. I have knocked out a few trail rides with him this week and was quite successful with him once I was able to mount him. Here's the problem: he spins the second I lay the reins across his neck and prepare to mount. It's not a walking off problem or a "little" spin at all! He spins hard and doesn't stop, even when somebody holds him. I've tried mounting against a fence, and that hasn't helped. The only way that I can, is if someone holds him by his bit from the back of their horse. That way he cant spin into the other horse, move forward or spin into me. And if you put a foot in the stirrup and rush to throw your other leg over his back, he's off for the races and at times will buck! He'll do anything to keep you from getting your seat. Now, I've had this horse vetted and had a chiropractor check him over. He had his teeth floated in April. He doesn't do any of this in an indoor arena- he stands perfectly still! I lounge him for fifteen minutes on each side and also try spinning him to disengage his hindquarters before attempting to mount with no success. This only happens before leaving the barn for a trail ride. Out on the road I can dismount and remount without any problems. He really seems to enjoy the rides and leads with excitement, so I'm thinking it's just a real bad habit/ trick that he's learned. Any other suggestions on how to correct this?