Before you start grooming a large, sensitive dog, you first need to understand why he has become so sensitive. Some pups just seem to be that way from the very beginning, which may have resulted from being roughly handled at a very young age. Others can become sensitive to touch as they age due to a number of medical conditions, or if yours is a rescue animal, it could be to past mistreatment. In any case, you should try to account for this in the way you handle your dog during grooming.
Your furry pal may or may not be aware of why he is so touchy when you try to groom him. He just knows that being touched in this way bothers him. Basic grooming methods are similar to those used for most dogs, you need to be very gentle, take breaks frequently, and at the same time use plenty of treats to keep your pup calm during the process.
Grooming a sensitive dog presents certain challenges, but the most important thing is to remain as calm as you can, be very gentle, and let him set the pace. This will make it easier for everyone. The best thing you can do is make each grooming as much fun as you can--consider it a great time to bond with each other.