Of course this is normal. When your son gets too over stimulated mentally, the way he is calming himself is by the rags. They make these pieces of material for kids to help them cope with the world around them. He can't help it, and I would encourage him to use them.I have a child in my class that uses a harmonica for his calming tool. And his behavior with his teachers is also perfectly normal. Does he have a calming space at school that he can just sit and veg out for a bit? Do they help him with his transitions? All of this you have to take into consideration. Have a behavioral specialist step in and give you and the staff some tips. Also, he's just like any typical 13 yr old with hormones. It's maddening yes, but he will be asertive and is just being a typical male, but without all the verabalization! Learn to deal with him as you would a typical 13 yr old.
I also have a 13 yr old son with DS. He has hundreds of dollars worth of toys but he perfers the squishy stretch animals. He loves to dangle and play with them. Is your son verbal? Does he have a counselor at school? Does he get violent with you ? My son will get others more behaviors than me. Yesterday school called to say he was sitting like a indian in his chair and refused to go to class. I drove in and said come on we're going to class. I touched his shoulder and took him gently by the hand. sometimes voice and touch help to make your request better understood. Also I disapline him by taking away something he wants. Like goldfish crackers. I tell him when you can , (do task) you can get some fish. You can message me if you like I dont come to this forum everyday.