Have you read any books on boundaries? (There is a good short easy intro by Anne Katherine.)
How about: "FIL I really appreciate your help with this project and I love you for wanting to help us out. I need to be clear though and ask you to listen to something that may be hard for you. I will be managing the project and its timetable myself. This is my house, my project, and I will have to manage it in a way that works for me with respect to my work and other priorities in my life. I understand that being a worker bee may be an uncomfortable role for you, so if you prefer to stop working on the project, I'll understand. It is really OK either way. I will love you either way. I just need to put a clear boundary between our R and my household busines."
