How do I solve this problem with my horse.?

Having trained many horses that I have owned over the years, I have encountered a unique problem. My gelding is nearly three years old I have trained him on the lunge, and he obeys everything to the "T" with just voice commands. Since the age of 2 he has had no objection to either the light weight saddle or the Bitt, in some ways compared with my other horses he has been too easy. The problem I have is when I mount him he refuses to even take one step forward no matter how much coaxing I do. I am not one to use the crop or any other violent methods as this never works. He really is a gentle giant, and it looks to me that he simply does not know what to do. I have even asked someone to try and lead him, (with me on his back), but alas he refuses. This is very strange to me as some horses will simply take off, the first time they are mounted in various degrees of Rodeo. Has anyone else encountered the same situation, any advise would be welcome. I do have another horse here but I am the only person who she accepts on her back, this meaning I cannot use the other horse to lead which may have been a solution, and there are no other horses near to me.