You don't neccessarily have to use ribbing for the neckline , if you cut the neck opening larger. You can find some great knit fabrics that that the stretch per inch and use that. Back in the "day" buying ribbing was not as readliy available and on the back of pattens there was a stretch guide showing how much a fabric needed to stretch to properly make the pattern work. it was critical for the necklines on many patterns as well as other parts of the pattern.
Another option is if he does not need all of the lenght of the shirt to use about 1 inch of the bottom of a knit shirt.