Karrie,
American Lake, just 10 minutes from Dupont, has sandy beaches where your son could build sand castles. Make sure to come down to Olympia for the annual Sand in the City event. It takes place by the Port Plaza from 21-23 August. Here is a link:
Hands On Children's Museum > Sand in the City® You'll be able to see amazing sand sculptures, and your son can play in tons of sand that gets brought in for the kids. The Hands-On Children's Museum hosts the event and also offers other fun kid activities.