Was it ever specifically mentioned in the book that he grew out a beard when he left for Ares IV?
It doesn't look like it. The only mention I see relating to facial hair is in chapter 9:
I’ve fallen in to a routine. Every morning I wake up at dawn. First thing I do is check oxygen and CO2 levels. Then I eat a breakfast pack and drink a cup of water. After that, I brush my teeth, using as little water as possible, and shave with an electric razor.
So he can shave, if he wants to do so. Other than this I can't find any other reference to beards.
(NASA used to have wind-up (spring operated) electric-style razors at one time. They were wind-up to eliminate electrical interference.)