Just some ideas:
What if he typed?
- What if in one window he typed out the distracting thoughts, and then in another window he typed out what he is suppose to type?
- Or create a draft that includes the other "clutter" that can be edited out
What if he wrote, have 2 pieces of paper in front of him. On the 2nd piece of paper, learn to write short notes/thoughts out on that one, and on the 1st piece of paper continue with the original assignment?
What if he spoke and someone else typed it out for him?
What if he had a recorder that could record his distracting thoughts and he could put those aside knowing they are recorded?
These aren't actual solutions but may help to at least figure out if these might help.
The act of writing, putting thoughts to print is not easy. Esp. if the brain is busy thinking about 100 other things.
One other thought is that, does he have issues with perfectionism also. That can really prevent one from starting. What should the first sentence be? What words should I use? What about these words? That doesn't sound right, etc etc... In this scenario, would need practice not working on grammer and spelling and just practicing dumping thoughts out. Perhaps because the act of writing slows it down and he can wind up assessing his own work. Perhaps start with audio recording first to practice mind-dumping. Then try using mind maps to organize thoughts, etc etc.
I hope I'm at least a little bit helpful. If not, that's ok.
Best wishes
