My son did a program through school called Fast Forward (maybe 4word??) It is considered a program to improve reading skills for most kids, but was done specifically to target processing speed and working memory skills for my son, who had a significant difference in his processing speed and PRI scores - similar to you child's. This is a brain training program, may be similar to the one recommended to you. I have noticed a great difference in his timed test math skills and his focus on completing some tasks. He was diagnosed 2e (pervasive developmental disorder) by a neuropsychologist in part based on IQ scores similar to your child's.
I did find the Fastforward program. Its on my list of things to research upon.Thanks. I am so glad to hear that it helped your child.