Let me start off by saying, I know exactly how you feel. I have an Aussie pup who loves bikes just a little too much.
First, start on leash. When you see a skateboarder coming, have HIGH VALUE treats (not the regular old treats, something amazing and unique to what they normally get). Give constant treats for quiet to begin with. Then, when your dog is staying quiet while the skateboarder, you can start expecting more. When you see the skateboarder passes, ask your dog to sit. Give constant reward as your dog is staying in a sit while the skateboarder is passing. When you have mastered that, ask for eye contact as the skateboarder passes. Once your dog is able to give solid eye contact while the skateboarder is passing, then start to walk past while asking for eye contact. Everything you do, you need to go slowly. Take it at the pace that your dog is ready for.
Only once your dog is PERFECT on leash, should you attempt off leash. Apply the same exact plan to off leash as on leash. Ensure that you have 110% control over your dog, because even if he is just herding those nips can hurt.