Yeah this is true, its already harder to train OD, especially for someone over 6'4. But i guess other teams has some success stories of training other players over 6'4. Taller players do improve in OD, just takes a little more time, but you should get a pop next week hopefully.
Are you single position training him? Depending on your trainer level, i don't think you should ever train a 6'5 player or over if you have less than a level 5 traininer in two position training. Sometimes though it takes 3 weeks to train OD, must be patient with that skill.