OP - one option you might want to look into is Wonders Childcare at Leland Community Center. They have a great NAEYC-accredited Pre-K which my daughter (May birthday so no cutoff issue) attended before starting at RHPS, and your son will definitely have kids in his class who will be going to RHPS after Pre-K (also Somerset, Bethesda ES, and Lafayette.) It's less expensive than many day care centers in Bethesda (so I would think certainly less expensive than downtown centers) and has before-care - not sure of hours - and after-care until 6. We plan to do that for my son who has a November birthday. More info at
http://www.wonderschildcare.org
PP, keep in mind that Pre-K at RHPS is need-based as a part of Head Start, so depending on income you may or may not qualify.