Ask HN: How do I get my designer to use Interface Builder?

I'm the founder of a tiny startup, with one or two other engineers + a part-time UX designer. I'm trying to get the designer to use Interface Builder to prototype UIs, and finally deliver them as storyboards. As opposed to PDFs, Sketch files or other formats that require a massive re-implementation effort. Some designers I've worked with earlier refused flat out, but I'm making the argument that it's not his productivity that matters, or mine, but that of the team as a whole. Have you gotten your designer to use IB? What are some effective ways in which designers and engineers can collaborate without each re-implementing the UI using tools they like but the other side doesn't?