I am not a doctor but I have decades of personal experience and research under my belt for issues such as this, also unless im reading it wrong you describe a kyphosis not scoliosis but thats moot as what im about to say applies regardless.

I have herrington rods in my back as a result of ehlers danlos and this story hits close to home few things Id like to bring up based on my own experiences.

Does he have Scoliosis/Kyphosis? or is that simply the by-product of something else like marfan's syndrome or ehlers danlows.

I ask because the muscle atrophy associated with either can definitely cause severe pain and can often be misdiagnosed as just scoliosis/kyphosis causing a poor treatment regime or the wrong surgery being preformed which can very much make things worse or hamper the healing process greatly.

I would rule out anything such as a connective tissue disorder that the scoliosis/kyphosis is a symptom of (as opposed to being the whole issue.) before getting any sort of fusion done to insure he gets the correct fusion, as the fusions for different underlying conditions can vary greatly.

Having said all that I would recommend a fusion 100% and especially when hes young because the bone grafts in a spinal fusion can be hard to have heal correctly once hes stopped growing which can lead to failed back syndrome and cause life long chronic pain or lead to further risky surgeries down the road when hes less apt to heal well.

If a curvature progress too far it can start to effect his organs such as his heart and lungs and thats very very bad.

And remember the fusion will only be the first step in treatment he should keep his weight down and exercise after its safe to do so working on building his core and supporting muscles. (Talk to your doctor about how best to go about this.)

Good luck all the best!