Is he using the keyboard for all his assignments, or is he writing them out by hand?
If he is writing them by hand, the mental effort required to manage the physical task of handwriting is undoubtedly going to decrease the attention available for content development. If this is the case, see how he does when he is using the keyboard for everything. He also may have developed a habit of saying the most he could fit into the fewest words before he had the keyboard, and he may have to make a conscious effort to un-learn this previously-adaptive skill.
Some basic tips on improving the use of details in writing:
Start working at the level of the sentence. Detailed sentences help naturally develop detailed paragraphs, essays, and stories.
Encourage expressive, specific verbs - strode, slunk, skipped, sauntered, crept, sidled, or bounced, for example, instead of "went" or "walked".
Encourage expressive, specific nouns - jalopy, rattle-trap, clunker, mini-van, coupe, limousine, roadster, or convertible instead of "car".
Using the old journalism stand-by of checking to see if you have answered "who, how, what, where, when, and why" can be very helpful to self-prompt to include more details.
Does the teacher have a rubric that she has shared with the students? If not, could she make one that explains what she is looking for for each letter grade? Sometimes just knowing what you are expected to do can help you meet the goal.
