When I was younger, I used to get similar symptoms in the spring and fall. My doctor eventually diagnosed me with
Fifth Disease, so called because it was the fifth virus affecting children that includes a rash. I believe the other four are rubella, scarlet fever, measles and scarlatinella.
I was told that I was exposed to it once and it just kept cycling through my body, hence the reason it popped up about every six months. Unfortunately, there are no treatments for Fifth Disease and I eventually just grew out of it. I'm not sure how that works, perhaps as I got older and had it more, my immune system could fight it better. I was eight when I had my first outbreak and sixteen when I had my last one. My outbreaks mostly occured on my face (making me look like I had been slapped all the time) and my chest and belly. They would last off and on for about a month and then reappear six months later. Nothing ever itched really badly, it was just puffy and a bit uncomfortable.
I don't know if that it was your brother has, but it sounds similar and might be worth looking into.