My daughter used to hide behind a long, heavy fringe of bangs secured with a hat pulled down low for a while and with that came a lowered head, defensive posture and mumbling. It looked like she was trying to hide in plain sight. She was having some anxiety in general and specifically, had been made fun of for the "wingy" curl that sometimes happened on the edge of her bangs and for her full eyebrows. Once she admitted what the issues were, we worked on getting her eyebrows and bangs into a look that she liked and could maintain and she started to show her full face again. It was just an awkward phase to get through.
It could be a "security blanket" type thing that offers a protective barrier to the outside world. if he wears the hood up a lot, it could also signal covering up something that we think is silly but that might bother him, like acne, hair he doesn't like, big ears, etc.
If he were hiding cuts or track marks or was unusually cold all the time like some drug users can be, I'd say that you have a problem. But given that you've taken a look at his arms and see him sweating like anyone else would in a sweatshirt in the heat, I think you can ignore this and it will likely pass, but keep an eye out for other behaviors that might signal anxiety that's at an unhealthy level.