He's probably going to want 52x20 if there aren't any significant hills around. A 52t ring should be easy to find. It's about equiv. to 42x16 if a new cog is cheaper. In flat, flat Chicago 52x20 is too spinny, but it should be okay if there are rolling hills.

Mind you this all depends on him as a rider. But I am not particularly strong and ride 44x16 on the flats.