English word construction seems like anarchy, but there really are rules—they’re just rules for all the different languages that english draws from, respective to each word. I learned to recognize the origin of words (from Greek, Latin, German, French, etc.) pretty early on, and it drastically cleared up the seeming chaos of how a word was spelled. Is your son getting any context behind the words? Otherwise I could see these exercises being as frustrating as trying to learn a modern symphony by ear with no knowledge of music.