My son has suddenly been labelled a defender

I’d be careful not to assume that because he enjoys defending, the label is harmless. At U12 players start building quite strong ideas about what they are “good at” and what coaches expect from them. If every match confirms the same role, “I’m a defender” can quickly become “I shouldn’t do the other things.” The useful conversation is probably with the coach rather than with your son first. Ask how they see his development across the season and whether he will still get chances to experience midfield or attacking pictures. He does not need to rotate every ten minutes for rotation’s sake. Some continuity is useful at 9v9. I’d just want the role to stay an opportunity rather than an identity limit. A player can enjoy being a defender and still develop the confidence to solve problems elsewhere.