Questiona6400 soccer settings? half my shots are blurry

Hi, new here. My son plays U10 soccer Saturday mornings and I'm using an a6400 with the Sigma 18-50, mostly in auto. In about half the shots he's blurry even when the background looks fine, and the sharp ones are usually when he's standing still. What settings should I be using? I don't really understand the AF modes. Played around with the menu but there are about five focus settings and I'm not sure which ones matter. I'd like to stay in auto if possible but I'll learn if I have to. Thanks