Does your son have any grapho-motor troubles? Have you had his vision checked?
I have seen scores like this for children with ADHD or learning challenges. The PSI relies heavily on visual processing skills, graph motor skill and concentration. While the WISC and the subtests contained in the test cannot by itself diagnose learning issues, it can raise red flags in areas of difficulty. My son's scores are somewhat similar (although his PSI is much lower), and he does indeed have ADHD inattentive type, graph motor issues and other LD's.
That said, the PSI is the index least indicative of a child's thinking ability. It is more indicative of how quickly and efficiently he or she can take meaning from input and show what he knows and make output. A slow processor is much like a super charged sports car stuck in rush hour traffic. Lot's of horsepower, locked in a bottle neck.
Since he has difficulty during writing, it might be prudent to evaluate for OT or vision issues. The difficulty staying on task and moving around could look like possible ADHD, but I would rule out other difficulties first. When kids are overwhelmed or struggling, they often squirm, avoid the task and become distracted.
How are his literacy skills? spelling? math facts?
