Having dorsal pain with resisted wrist flexion doesn't make huge sense. It sounds like you've tried alot of things... except removing him from sport and doing a higher level of strengthening... How viable is that? - e.g. how close are u to end of season etc. With having no injury to bring on the problem we assume it's come on from his sport - removing the sport and undertaking a much harder strengthening program may be what he needs to return pain free to sport.

You can look at some mobilisations of the wrist if he has pain inside the wrist and assess that, check for neural tension issues as well... and if possible get a second opinion with one of your supervisors if they are nice like that.

If it is still plateauing and the pain is significant enough get some further investigations done in terms of xray and US.