I will be the first to say that it is not a good idea to go into a partnership. Now if you wish to sell your shares of the company then you should sell them. He can't lock you into a company that you no longer want to be in. I think his problem is that he doesn't want you to come out against him with your own company. But you have to ask yourself do you really want to give up a successful company. If you do then by all means look for someone whom will be a good fit and then sell the shares. He can only block you from selling them for so long. Good luck