Hi Bill!
I've had hamstring lengthening surgery (one of about hald a dozen surgeries total) so I may be able to shed some light on this for you. I had the surgery when I was 13 and am about to turn 22 in October. Your son sounds like me at his age. I was very active, though I stopped wearing AFOs when I figured out how to take them off myself
As far as lengthenings go, hamstrings are a piece of cake. By far the least amount of pain and fastest recovery time of any ortho surgery I've had. Your son will probably be in long leg casts for 6-8 weeks and imobilizers for another month. The only down side for me was that my casts kept sliding down and the plaster cut up the front of my ankles really bad. So much so that my doc removed the casts early. Also, I got bed sores on my heels from being in a hospital bed for a week. Those were the worst (I ended up staying in the hospital 5 extra days because anesthesia makes me very nauseous and I refused to eat). Other than that everything was fine. Once I switched to waterproof casts I was able to shower, go swimming, etc.
Pros:
I could straighten/ fully extend my legs after the surgery.
Lengthening is probably the reason I can still walk today.
I grew about 2 inches.
Cons:
I lost almost all the strength and stability in my legs. Presurgery I was very into extreme sports (skateboarding, surfing, bmx). I can't do any of those things now because I'm not strong enough and my balance sucks.
It took about a year for me to be able to fully bend my knees (such as crossing my legs indian style).
I developed lordosis.
My knees hyperextend.
I experience much more fatigue (but I am much older now... that has to be taken into account!).
My hamstrings are contracted again, but they can't be lengthened again. You can only lengthen so many times.
I shrunk 2 inches when my hamstrings contracted (so I came out even

).
Now, even though my cons list is much longer, that doesn't mean I think it's a bad surgery!
I think you and your son will have to consider the degree of contraction. How crouched is his gait? Do you think it will get much worse?
As I said, if I hadn't had mine lengthened I would probably be too crouched at this point to walk.
Is he willing to (possibly) give up some strength and/or stability? I had adapted to do all the things I loved (like skateboarding) with the way my legs were before surgery. I was horrible, but I could do them. I think it was just too much of a change for me to learn all over again. This may not happen to your son, but I think he deserves a fair warning!
At this point in my life I still walk completely unaided, though I have been trying to get a wheelchair for over 2 years now to deal with fatigue. Looking back I probably would still have the surgery, I just might have waited a few years so I could have a little more time doing what I loved.
Hopefully others will be able to share their experiences!