The school (a Christian school)that my boys go to has a financial aid program, but I'm not sure that it applies to the preschool. Now they do have a "kinderreadiness" class that is considered part of the grade school, so that may qualify. Anyway, you may want to call and ask about what may be available for you son.
It is the North Texas Christian Academy. Their number is ###-###-####. Here is their website: http://www.northtexaschristian.org/
Good luck!!
P.S. I know that another school my older son went to offered aid also (it was also at a church). So you may want to ask any program that you look into (especially church programs) if they have financial aid!!