If he has a 5:4 monitor, 1280x1024 will give him the proper aspect ratio unlike the others.
Unfortunately Kingpin does not support non-4:3 resolutions.
I have a standard 17 inch monitor (Acer AL1715).
That is a 5:4 monitor, LCD monitors are usually 5:4, 16:9, or 16:10. The old CRT monitors, which were the standard when Kingpin came out, where 4:3.
Your only option in this matter is to run the game at 1280x960.