There are two programs for low-income children--the Montessori preschool program (which he has virtually no chance of finding a slot for at age 4, even if your income qualifies) and the Virginia Preschool Initiative (VPI) program, for low income families. His classmates in the VPI program will be almost uniformly ESL learners due to the demographics of Arlington, but Arlington schools are excellent and I'm sure the program is at least decent. I have heard the Montessori preschools are excellent, so you might want to go ahead and get on the waitlist just in case a spot opens up.
Note the low income cut-off in Arlington is pretty high, 80k or something like that.