Looking for- older book, earth is a penal colony and a boy visits medieval Normandy, uses his tech to help the Norman's in battle

[Bartrick]

My uncle gave me this book to read when I was in 7th grade (1991) and I fell in love with it. It was an older book at the time so I'm guessing it was from the 60-70's. Unfortunately he has passed and his kids don't remember the book. Earth was a galactic penal colony and all of the criminals see sentenced here had their memories erased and had to start over as cavemen. That is why earth is so far behind all of the other humans in the universe. In the book a boy visits earth (illegally I think), during the medieval times ending up in Normandy and meeting/living with the Norman's. He uses his advanced technology to help them in battle. I really can't remember much more than that, but I know it was my first sci-fi book and I loved it. All of my friends who are into sci-fi have can't figure it out and Chat GPT couldn't find it either. Hopefully one of your pros can help me! Thanks in advance