Niki, he does seem too young to be giving up on naps so it might just be a phase. In the meantime I would suggest a really early bedtime... as early as 6pm to give him chance to catch up on sleep. On the days my dd doesn't nap (the 2.5yo) she's in bed by 6 & she wakes at 6.
So for now I think an earlier bedtime shoudl help your friend & his ds.