Somehow, the glove size that you mentioned shouldn't present a problem. However, like I mentioned, looking at his managing the glove does look awkward. But then again, my perspective does have its limitations.
I'm curious though why his coach would recommend a larger glove? Did this coach tell you why? Did this coach point out certain things that your son should or should not be doing?
Could you post some video of your son just playing catch? Nothing special with respect to a pitcher's motion, just some relaxed movements on your son's part. Try a distance of no greater than 40 feet. Let's see how he moves without trying to be letter perfect pitching wise.
I should mention that at 9 years of age, your boy has a lot of muscular maturing to do. His motor skills and muscle memory will start to kick in during is early teenage years and you'll probably see noticeable improvements with his coordination and other things. So, just be a little patient with his progress. Pitching is one of the most demanding mental and physical things asked of the human body. If it were all that easy, I would have been out of job years ago.