You may to have shave him. I'm a groomer and some sweaters are notorious for matting hair. If he is badly matted all over it might be best for him. Some groomers including myself refuse to brush out badly matted hair. It's just Torture to the dogs and they are likely to recieve Brush Burns.
He will survive with short hair. And be sure to take his sweaters off daily and brush him when it grows out again.
I hope this Helped, Good Luck.
