I have my students do a vowel drill to warm up for therapy: their sound in the center of a "circle" and around the circle are the long an d short vowels. So they say "kay, key, kigh, ko, ku for long vowels and kae, keh, kih, kah, kuh for short vowels. It might help your student if the syllables they're saying are for drills and have no meaning. Don't know if this actually addresses your problem or not. But it's a great warm-up exercise that helps kids with reading as well.