Demand w/out a contract?

Two years ago, my father-in-law gave me funds to help start a business. I’ve always considered it a loan with plans to repay him. The business is not making the money we expected and our relationship has suffered because of other personal matters. Now, we have the business on the brink of a big turnaround and he tells me he is formulating some sort of plan that likely includes radical changes to the operating agreement I have with my partner (my wife is a third partner). I also believe if our relationship continues to sour he could eventually demand his money back. I still plan to repay him but since we never had a contract or any legal document, other than a check, with his name on it, does he have any legal grounds to take the money or part of the business?