Hi, we had a similar situation when we came in 2015. Our daughters birthday is 22/07 so she was the youngest in Uk. Back then she was in year 4, and moving here she was meant to go to y3. However we insisted she'd go to year 4, as she wasn't happy about going to a younger grade. It has worked out ok, she's a bit behind with her results, but socially at the moment she fits well. She's starting y7 in January and is very happy and exited. Just do what your guts tell you, if you want him to stay in year 3 just ask the school