Sounds familiar, too familiar... oh yeah... that's stil how my writing is. I can't sign my name the same way twice.
Does it improve if he goes much slower? What happens using the other hand? Is it better with one eye or the other closed?
I've always had depth perception issues. I did figure out about 10 years ago that I've been holding my pencil wrong all my life with my index finger wrapped around it. And I only figured it out, by seeing a picture with an explanation. I can't pick stuff like that up from watching other people.
Don't know if any of the is helpful... but ... shrug