Installing a 3 phase panel on a single phase system is a clear violation of 110.3. The title says it all-- "Examination, Identification, Installation and Use" That is all you should need to say to him. But, try 110.3 (A) (7) specific use, and 110.3 (A) (8) practical safeguarding. What if someone ran a multiwire circuit off of the same phase? 110.3 (B) seems pretty clear, also.
