Hi L.,
A private school might not necessarily have the resources to help your child with a language delay. I understand your trepidation regarding MCDPS with its reputation but you would be surprised at the services available to children who need extra attention such as speech pathologists, special area teachers, and counselors. Additionally, remember that public school teachers must be certified, whereas private school teachers do not necessarily have to be. The most important thing, however, is that whatever school you choose you remain on top of informing them of your child's needs and making sure they are helping him. Go to the schools (even the public one- make sure they have pre-k!) and get a feel for the campus, ask what they will do for him in regards to his language delay, and take whatever advice other moms who have had experience with those schools can give you.
Best of luck,
M.