There are lots of great blogs out there with kids' tactile, outdoor, and art activities. At 3.5, my children also were playing board and card games. There are many good ones now that did not use to exist, even for nonreaders. They were also big into construction toys then. Both of them liked imagination play but it was not their main play mode. What about books on tape? This bought me many hours in the pre-reading years.
If you are home with him all day, and he is not in preschool, it may help to come up with a list of activities and a very loose daily schedule. I had times when I needed to do this and times when I didn't. OTOH, at that age, some kind of structured group activity MAY be a nice break for you and good for him. (I don't feel this is at all necessary for SAH kids before 3 or so, but at 3, I think it's a nice idea if the child might like it, and I think all kids should try it out before school if going to school.)
