First take a look at your own flying it may be that some of the things he is doing are not a problem to flight safety but just different to your SOP,s remember just how meny ways are there to skin a cat ? !.
Having looked at the above i suspect that his flying is a mixture bad habits and mental overload i found the IMC the hardest thing that i ever did in my flying training it was far harder than the IR because it was all new and the IR was just a lot of practice so the first thing is to get his basic IF to a very high standard so it becomes second nature to him.
Then start increasing the workload to cover the airmanship and flight management im sure he will respond if he has a solid foundation of basic IF and fully understands and uses the selective radial scan.