Your son is doing well according to the statistics. Surviving the first year is the biggest hurdle. Many patients don't make it past the first month. Once they've passed the first year anniversary they are probably more stable. Urinary tract or upper respiratory infections are the main cause of death for many spinal cord patients at any stage.
There aren't very many studies long-term results after diving accidents. Recently doctors in Athens, Greece reviewed 20 patients who had diving accidents over the past 34 years. They looked at neurologic status and improvements over a 10-year follow-up period.
About half of the patients available for study made some gains in the first five years after the accident. The other half was unchanged. Ten years later one patient had improved neurologically. One was worse.
More and more scientists are finding out how changeable the nervous system is. They call this idea neural plasticity. In the past there wasn't much hope for recovery after the first year. Today there is greater hope than ever for gains in recovery. Improved treatment early on and new discoveries about the nervous system may offer some breakthroughs for your son.