mickeyM: Please read and learn from the story how Paul Allen departed from Microsoft. From Wikipedia [1]: "In 1983, Gates tried to buy Allen out at $5 per share but Allen refused and left the company with his shares intact." So you can leave and retain your shares. If Jim continues developing existing app under a new company, he effectively steals the code you co-own... Remember that you also invested your work into this app (sketches & app flow)...
[1] https://en.wikipedia.org/wiki/Paul_Allen#Microsoft