Gates of fire is splendid indeed. Spartans are most interesting warrior breed to read about in historical fiction, them and the Mongols (Samurai's too, mainly Miyamoto Musashi).
If he likes Roman era, then he will be spoilt with books to read, I read 30-40 historical fiction books about Rome's republic/empire days, its way too much to choose from hehe. More political ones are more interesting (about Cicero, Tiberius etc), even if Legionary ones can be good, if you can avoid some of the gazilion hack writers that write in that action genre section of Roman fiction.