Welcome to YT! And congratulations on your little boy
Stedmansmommy has said it well, please don't get angry at him. He will feel your frustration, and that will make it harder for both of you. He is just a puppy and this is completely normal behavior. My Yorkie took some time to get used to the brushing, and she had a tangly coat as a pup. I did keep her in a puppy cut from 6 -10 months and then starting growing it out. But brushed her every day (and still do). What you want to do is get him used to it gently and slowly. Take a brush and just brush one or two strokes on his back, reward him when he stays still. Keep the sessions short. Do it daily. Before you know it, you won't remember when it was that he was growling and squiggling when you tried to brush him, lol. Just keep progressing and rewarding him for letting you do it. Even if you do decide to keep him in a puppy cut, keep brushing daily, then when the time comes that you want to grow it out, he will be used to it.