Then before I leave I just have to tell them I am going, and when I think I will be home. Visit a skateboard shop in your town and talk with some of the salespeople there. Usually these people are young and skateboarders, too. They can tell you a little about the local scene, who does it, where the kids go, how safe it is. Make sure your son is wearing protective equipment.
Accidents happen and head injuries can be serious.
Investigate a skateboard camp online. They do exist and might be a good way for your son to spend a week or two this summer where he can pursue the sport to his heart's content. Ask to go and watch him skateboard. Visit some of his favorite spots on your own so that you can reassure yourself about where he is. This, too, will pass.