I would be worried too at that age. Even if your son likes playing GK, insist on him playing 1/2 the game on the field (e.g., 1st half GK, 2nd field player, preferably a Forward or attacking mid - most successful goalies were Forwards prior to committing as GK). Ok to specialize by U13 or thereabout, as each position becomes quite tactical at high level.
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Well, the added problem is that it's U9 so his squad has 12 kids 7 on field at time. As a keeper, he's a starter. As a field player, he usually plays winger and is a middling player for his middling club. It's u9 so it's early and he's gone from being an AYSO player ranked 1.5/5 his first year at U6, to a 3.5/5 at U7, to 5/5 at u8, to a rookie club year, so there's a lot of upside potential. But right now, he's a natural at keeper, though only meh on the field which makes it difficult to demand playing time with the other benchers competing for the 6 non-keeper slots as well.