Bro don’t do it. I had to reread this a few times to decipher the chaos

- You’ll be stretched thin between your two jobs

-51% = Not really your start up but his aka your new boss

-If the start up will fail if you leave, it means you shouldn’t part with 51% if you are a key reason why it functions

-How successful is his current company? Why no valuation or delay in payment?

- Usually start up founders don’t take a salary or take a very smallish salary in order to grow the company, is there a reason why you are paying yourself so much it will run the company in the red?

You need to get some lawyers in on this, this sounds like something that will have you on a psychologist couch if you don’t get it taken care of. It also sounds like you need to make a choice between the start up and your job because its stretching you thin

Looking forward to the answers here