Why is 13 year old sons arm more hairy after having it in plaster for six weeks after it was broken and cast removed?

While your son's arm was in the cast, he wasn't able to move it around very much or wash it. The hair on his arms was able to grow without being rubbed off or washed off. You might notice that most men who wear short-sleeved shirts often, have very little hair on their upper arms. This is because it has been rubbed off, or at least thinned out, over time, by the short sleeves. The extra hair will probably thin out after a few months.