It's just a fluid buildup in the knee. Taking a look at his injury history, I can't see any specific injury that would've caused this problem so I assume he's got arthritis in the knee (I'm not a doc, don't trust me on that!). I think all they can do is regularly drain the fluid when it starts causing a problem with mobility.
I imagine that it'll only get worse as he gets older. Realistically, we just need to hope that he can trudge through the next two years and then cut our losses.