Personally, I wouldn't touch it. The first danger sign is that "if the investors see potential in what I develop in phase 1". What an investor will look for is a solid business plan, backed by thorough research into the potential market. Has this guy done that? Never mind how great the application will be. The important thing is the market.
If he has faith in the product; if he is sure the market exists; if he is confident that he can successfully sell the product, then he should put his money where his mouth is, and pay a decent fee to the developer. If not, then it shouldn't be up to you to share the risk.
As for asking for a cheque in advance and not cashing it unless he proceeds, that seems fraught with danger. What if he spends a lot of money getting the business off the ground, and consequently goes bust before you can cash the cheque? What if he gets into a dispute with your for whatever reason, and decides to stop the cheque? What if - and this seems the most likely scenario - there are endless delays and prevarications in starting phase 1, and by the time he decides not to, the cheque is out of date.
Above all, I wouldn't go for it because I see no reason why I should take the risk. I would take the view that I am a software developer; I will write do the best job I can with the software, and I want to be paid accordingly.
But that's just me. Different people have different attitudes to risk and reward. If you can accept the risk, don't let me put you off.