I'm not sure what the purpose of the assignment was. If it was to write a thesis paper, then I agree - he didn't succeed. He has a statement that could be a thesis right at the start, but then he just narratively tells the story of the battle.
As a summary, it's fine. I don't see it as being "behind" other kids this age. His grammar, mechanics, and vocabulary are all fine, but he has very repetitive sentence models, so if I was going to work on something to improve the paper, I might work on that. The organization is fine overall.
Again, I don't know what the intention of the writing is, but I think this type of writing - tell me about the ___ writing - is hard to make engaging. It's not an engaging paper, but summaries aren't engaging for the most part. I think the best thing you could do would be to make him write a thesis paper on it - prove that the battle was the most important one in the war or was a turning point or whatever. Or, it would be to change the assignment to be something with a clear audience or format. When I've wanted my kids to write a basic summary, sometimes I've had them write it as a children's book text. But you could have him write a newspaper editorial from the time about the battle. Or he could get creative and write an eyewitness account from someone there or a diary entry from one of the principal players. All of those formats are automatically going to have more to play around with in terms of the writing because they have a clearer purpose than this "summary." This is basically a narration.