My son is 6 and in the past year he's enjoyed swimming, soccer and TKD. All one at a time, though!!! Don't schedule more than 1-2 things for him right now.
We take swim classes through Fairfax County parks and rec now (winter). Their classes are great and at many different levels. Soccer league was in the fall through McLean Youth Soccer and was also great. Jhoon Rhee has locations in Arlington and Falls Church, they have a great knee-high ninjas program.