The fact is you've only just launched, you've not proved the market for the app actually exists yet, there are still technical problems to overcome,
and the hard part of running a startup, eg getting customers and scaling up, hasn't actually started yet (feedback means very little; it's all about actual sales for real money).
You could just walk away and keep the shares, but as you're a minority shareholder your stake will be diluted to approximately nothing fairly soon.
You're in a really bad position here. If Jim wants to be a %^&% about this then you're going to get screwed.