What could this be??

I noticed this rash on my sons arm. I pulled his sleeves up for him to eat and noticed it when I was cleaning him up. He had an organic oat bar and broccoli, both of which didn’t give him any other symptoms anywhere else and haven’t in the past. It is a bit scaly feeling. What could it be?

He also had some sort of hives looking spot on his knees this morning also, when I went to change and dress him, but he wasn’t on his knees prior to it showing up. ??