Um, six years old? Talk to his parents? I mean I assume you have since he is allowed around you. Ask him if he is doing that to them and others and if they know why. Make sure they know he does it. Or you could get accused of stuff you know. Even if you don't touch him.

I will only guess he likes the sensation of skin touching skin. When it was colder and you had more clothes on, he couldn't do this. A lot of autistic people have touch stuff they really hate and some they really love.

Me I hate skin to skin. I love soft animal fur. I love sand. I love certain fabrics and completely hate others. Some can even go into a trance like state keeping touching the things they like.