S.W.
Hi A.,
My four year old daughter also has cerebral palsy. We do hippotherapy at a place called therapyworks in Boring,OR. There is also another large place called Forward Stride in Wilsonville (actually I think they recently moved to a bigger location in Beaverton). We haven't been there but I have heard good things about it. There are also places that do riding for kids, but from what I have found there are very few that do the hippotherapy program.
Good luck to you and your son. I know how challenging things can be sometimes.