Just go based on the letters and the function.

Does he think he's more E or I? Do the same for all the letters. Also, compare the functions? More Se or Ne? Etc.

You can also take all the type descriptions and break them up sentence by sentence (or however much text puts across one meaningful facet), and have him score himself on each one-- yes, or no. Total the scores across all the types, and whichever one is highest is a good bet for the one to go with.