Hi Vanessa. If he is over 6' 5" tall, he should make someone else hold his guitar for him!!! :silly: :silly:
Actually, if your student prefers to play in the seated position, have him move the guitar around and find a position that feels most comfortable for him. Once he finds that position, he can have a guitar rest made to fit his body and his guitar.
The guitar strap is a very good suggestion. Rick's suggestion for him to play a JUMBO is also good. Take him to a guitar shop and let him try out a couple of them and see if he likes them.
Bill