At each visit, I ask the therapist to tell me the two top things that I should be doing with my boy and I make sure that I do those two top things every day at least once a day. I keep a running list on the refrigerator so that the whole family knows what we are working on. It ends up being 6+ things a day and is time consuming but it is something that we all tackle. My boy has big sisters who also work on things with him. In fact, you can specifically ask the therapists for ideas of things that the big sister can do with him. I think that siblings can be great motivators and quite patient and be a great resources in working with our kids.
Ask the occupational therapist for things that you can do in your day to day interactions that will help strengthen your child and compensate for his weaknesses.
Sensory Integration therapy
I think that we all need to consider our children to be at risk for autism. I also think that the sensory integration stuff would benefit all kids, but especially our kids. I don't think that there is a specific article on SI and PWS, but our kids would classically fall under SID based upon their hyporesponsiveness and low tone. If you look a their genetic makeup, they are missing a gene that promotes neuronal outgrowth. I take that to mean that they need as much help as we can give them providing a stimulating environment to promote neuronal growth. Make sure that your OT knows that you want SI therapy for your child.
Read more about sensory integration therapy at Healing Thresholds