I’d first look at working memory–can he juggle all of the skills necessary to write, and do them simultaneously, or does he “forget” one or more of those skills?

If it’s not working memory, I’d look second at  ADHD, inattentive type. Can he initiate, sustain, inhibit, and shift his attention at the right times?

Finally, I’d look at emotional aspects. Is anxiety talk shouting over the youngster’s strong verbal strategic talk? Is he preoccupied for emotional reasons? Is he stubborn? Bored? Too worried about what others might think? Doesn’t see how sustained effort can bring results and feelings of competence? Is it too much screen time so real life has lost its excitement?