One of the great things about the Bull Mastiff is that while they tend to be exuberant when they are puppies, most calm down and become somewhat docile as they reach adulthood. But don't mistake this for a lack of protective instinct, your pup will reach around 130 pounds and will jump to his family's defense fearlessly. The good news is that Mastiffs don't need to be groomed very frequently. But, if you have a dog that becomes anxious when grooming time comes along, you need to work with him to teach him grooming is not something he needs to worry about.
Your Mastiff may be a bit wound up as a pup and wants to play far more than he wants you to give him a bath or brushing his coat. In fact, chances are that unless you started grooming him from the moment you brought him home, the tools you plan to use and the bath water are likely to make him quite anxious. It is up to you to show him he can relax.
The only time you are likely to have a struggle grooming you Bull Mastiff is when he is young. As these dogs age, they tend to become very calm and relaxed. Take your time, make the whole experience relaxing and enjoyable for your pup and you will find that grooming time stops being a hassle and starts to become bonding time.