Under Armour, also called rash guards, rashies, or a surf shirt, is something you want to try if your child seeks out proprioceptive opportunities such as tickles, hugs, and/or any kind of pressure that squeezes the body.
Under Armour is a tight fitting sports shirt that is worn to help regulate body temperature and dryness. The material is a lycra-type fabric that fits snug on the body and feels silky.
I had a student who wore a vest that I could velcro onto him over his clothes. It was the same type of stretchy lycra material that gave a squeezing sensation. We kept it at school and used it during work time to see if it would help him calm down. He loved it so much that if I would forget to put it on him in the morning he would go and get it from the closet and give it to me to put it on him. It definitely seemed to make a difference and he seemed calmer, more spacially organized and happier.
Here is the link to the Under Armor website but there are plenty of other options on the web. Body Glove, Quick Silver, Billabong, Roxy Roxy and O’neil also make cute shirts. Just look for rash guards and surf shirts and make sure you get a size that is snug fitting.