Does your son already show some promise at training - where does he usually play, or is this all pretty new?
Usually at this age, the choice of position depends on a few things. First, physical characteristics: a tall kid often becomes a goalkeeper or defender because it gives him an advantage. Fast and agile kids are usually tried on the wings or in midfield - they can push forward and fall back quickly. Those who are more technical and calm are put in the center of the field or in attacking positions.
But the main thing is to look at your son's own preferences. Does he like being the center of attention and scoring? Maybe he's a striker or winger. Does he like organizing play and seeing the whole field? That's a midfielder. Does he like defending, intercepting the ball? A defender or holding midfielder. The coach usually sees these tendencies better than anyone from the outside, so it's worth listening to his observations, but also take into account what your son himself wants - at 8-10 years old, motivation matters more than the perfect role.
At this stage, don't get too fixated: many kids still try different positions for several seasons before they find theirs. The main thing is that he enjoys it.