Hey Monkey. I have this and I am a little older than your son, 27, but still young. My fracture goes all the way across the L5 pars, so because of this, that chunk of bone is hanging out on it's own in there and my L5 vertebrae is slipping forward.
For conservative treatments I have tried: Physical Therapy (2 different programs), chiropractic, 3 rounRAB of oral prednisone, and 2 epidural steroid injections.
All the tests I have had done include: an X-ray, MRI, CT scan, and a discogram.
They will most certainly do more thorough testing on your son to get a really good idea about what is going on, how severe his condition is, and if any discs are responsible for his pain. Then they will always try a more conservative treatment approach before doing surgery, unless it becomes an emergency situation (such as he looses control of his bladder or bowels, or if he looses all sensation in a leg..) If he can deal with the pain, he can put off surgery for awhile until his back becomes too unstable or the pain becomes too much to handle. It is a nasty operation.
My pain has been going on since last spring, and has progressively gotten worse over the course of the last year to the point that it is very severe, so I am having surgery in a month. The surgeon will use bone from that free floating chunk of bone, and roRAB and screws to fuse my L5 and S1 vertebrae together. I will have 2 incisions, one in my abdomen and one in my back. Going through the abs makes it easier to reach the disc that he is going to remove.
If you have any more questions, ask away!