I know this thread is old, but I felt complelled to put in my 2 cents
I believe that some people with CP can be mentally retarded, but I think that MOST of us are either average to above average intellegence even those who aren't able to speak in the regular fashion.
As for me, I am fortunate in this area. I was born at 2 lbs 1 oz, and was born about 11 weeks before my due date. My parents did start me in therapy and preschool very early to get a "head start." (I started preschool at 3yrs old), and I believe it benefitted me a lot. I don't know how they thought I'd turn out mentally, but ended up excelling in the academic side of school, and I'm currently finishing up a degree in computer science. Its always fun to see the reaction on people's faces when they ask me what kind of classes I am taking. (now the hard part... I just need to get a job

)
Whatever you do, don't sell your son short. He might not ever be able to do a lot of the physical things guys do, but he could do well on the academic side of things. I saw several kids in elementary/ high school who were in Special ed classes when they were very intelegent. I think somewhere along the way their families didn't expect much of them, or babied then so they never had a drive to learn anything more than the bare minimum. It was sad.