Use the "crawl before you walk" metaphor and tell him to design it to 2d first and when that gets going, you all can look at 3d. You'll never do the 3d version

Use the "crawl before you walk" metaphor and tell him to design it to 2d first and when that gets going, you all can look at 3d. You'll never do the 3d version with him but maybe he might do it solo... You guys might actually accomplish something. Valuable lessons to be had in risk containment, testing ideas, etc.