There probably isn't much you can do apart from keeping it clean for now. Does it hurt him when he does a wee? When my eldest was 6 similar thing happened, whenever he would need to wee it got very swollen and was very painful for him. He ended up needed a circumcision (not saying that's going to happen for your DS) but definately take him to your doctor tomorrow. Hope everything turns out fine for your DS.

Regards,
Dianne