I had two injuries to my right knee when I was 11 and 14, and then 4 surgeries each year after the second injury. I tore one of the ligaments and my knee and then repeatedly tore cartilage.
It's odd that the consultant suspects a cartilage tear but it didn't show on the MRI. It might be a small one. The menisci don't have a good blood supply, so tears don't tend to heal on their own. They can cause catching, locking, clicking type pains when the knee bends. Minor keyhole surgery is usually required to remove them.
What are the knee-bends? Is it weight-bearing, like a squat? Weight-bearing exercises can be very good for building strength and also balance, which help the knee to be stable and prevent further injury. However they can also aggravate an already present injury by putting too much pressure on the joint. If that's the case, he could switch to non weight-bearing exercises where he lies on his side/back or is sitting down.
pronation, buckling knees and gait
If this is causing the problem, it can be tiring to solve, because the solution is working on his posture and how he walks and his muscle strength, rather than wear this brace/have this surgery. I resisted my exercises for months at a time because I wanted a quick fix, or they were boring, or I thought it was better so I stopped them early. It's difficult at that age to stick to something diligently. But it's important he keeps doing them, and finds a good routine with the physio that doesn't cause more pain.
It helped me to sit on the edge of a chair and raise my lower leg so it was straight. My kneecap would slide on a bit of angle, because I had general muscle weakness and problems with the kneecap sitting in its little track properly. I had to focus quite hard on tightening the VM muscle to keep my kneecap straight, rather than defaulting to using the other quad muscles which was easier. It took maybe two months before I could do it properly. It's not like tensing your calves or abs where it happens naturally; it's difficult to isolate that muscle and tense that one rather than the others. But doing that allowed me to keep my kneecap straight as I bent and unbent and helped stop further problems.
Building lots of strength, especially in his thigh muscles is really important. The joint needs the surrounding muscles to protect it from injury and overuse strain. If the muscles are strong, the knee will move soundly and smoothly. Balance is also really important, else the joint will be wobbly and unstable. I used to stand on one leg while I brushed my teeth, or wobble about on my brother's skateboard. Then the next times I would play sport and twist on my knee, or have a fall, my muscles had the habit of tensing and supporting the joint, rather than just being blobs of leg and leaving the knee to twist on its own.
Sorry that this has turned into a bloody essay! Not sure if it's helpful.