Before we go into how to shave a big dog, we need to cover one myth. Many dog owners are under the mistaken impression that shaving a large dog with long hair in the summer is a great way to help them stay cooler. The reality is that the ASPCA and most vets all agree, this is a bad idea. That thick coat of hair you are sure is making him hot is actually insulating him from the sun and summer heat. The only time you should shave your dog is when your vet recommends it or if the mats in his coat have become so severe cutting them out is not an option.
Your dog needs his coat to help keep him cool and to protect his skin from the sun's UV rays. Just like you, your pooch can suffer from sunburn, especially if you shave him right before summer. It can take a long time for many breeds' coats to grow back in. You may find regular brushing to be a far more effective way of helping your pup to stay cool.
Keep in mind that if this is the first time shaving your dog, he is likely to be scared. Take your time, go slow, talk to your pup as you go, just like your barber or stylist does while you are in the chair. In time your dog will come to accept being shaved, but this doesn't mean he is ever going to like it. Finally, be sure you check with your vet to make sure shaving your big fella is not going to cause him serious injury.