dear Rosie,

Is your young patient hypermobile, or is it juist this joint? This could make the treatment goals different.
I agree with Iliastoles that coordination,strength and stabilty probably needs to be enhanced. Consider the fact that this young persons body is probably in a "wrong" posture for most of the day.
I would suggest elastic taping (Cure-tape, KinesioTape,Kinesiology Tape, K-taping) on the joint. I would suggest applying a little tape around the joint. Make sure you tape the dermatome belonging to the joint. This sensory imput/ stimulus does not immobilise at all and could be of some help.

kind regards

Esther