Brushing a scared puppy can be a sad experience for you and a traumatizing experience for your little guy. There could be a myriad of reasons for your puppy to be terrified during brushing sessions. The only thing you truly need to understand is how to make him feel comfortable. Conditioning your puppy to enjoy brushing and understand that he will be rewarded throughout the process will help get him to a place where he not only tolerates brushing but is no longer frightened. Take brushing strategy to a new level by turning your grooming sessions into training opportunities that include the same rewards your little guy gets for behaving well any other time.
You may not know why your puppy is scared of brushing, but he could have had a bad experience in the past or it could be simply that he doesn't understand what's happening because he's so young and hasn't been handled enough in such a way. Consider reward-based grooming sessions so your puppy focuses more on the good instead of his fears and anxieties.