Your Saint Bernard is indeed a saint. He is a gentle, kind, loving giant beast who is eager to please and just wants to be near you. However, if your Saint Bernard is not used to being handled on a regular basis, he might be a little wild, anxious, and apprehensive before grooming sessions.
If you need to calm your Saint Bernard before grooming you need to make him comfortable and relaxed. Allow him to sniff all the grooming tools. If you're using something like clippers, let him listen to what they sound like and feel the vibration of them on his skin before you clip him, and reward him during your grooming session, not only at the very end. Keeping your Saint Bernard’s attention isn't hard to do with a good belly rub and a tasty piece of cheese.
If your Saint Bernard is anxious before grooming sessions, it's possibly because your Saint is not used to being poked and prodded very often. To get your big guy used to grooming, poke and prod during times outside of grooming sessions. Practice with conditioning behaviors will help him to relax.
Your Saint Bernard is a big fluffy teddy bear. He is going to shed a lot and he will be dirty from food, drool, and anything he can get into. He will need to be groomed often. Calming his anxieties early on in life or in the grooming session will make grooming tolerable and even rewarding for you both.