M.C.
There are also 2 Primrose schools in McKinney. I have heard mixed things about them, so do your research, and see if they're a good match for your son. I have a couple of friends who work at McKinney Montessori, which is also good, but I don't know if they take kids for only a morning or two a week. We're thinking of sending our DD to Holy Family School in McKinney next year (fall 09)--it's affiliated with our church, and we have friends who highly recommend it: http://www.hfsmckinney.org/history/index.htm
They start at 2 y/o, and you can choose 2 days/week, 8-2:15.
I also know that the Lutheran and Methodist churches on Virginia just west of 75 in McKinney offer preschool, but I don't know anything in particular about them.