You can doubt it all you want. My kid impresses his classmates with his reading and writing abilities. And yes, I am familiar with the various concerns people have regarding kids' faces on social media. It's a complex enough issue that I don't have a hard and fast rule about it and we're still discussing it with each other and him.
And yes, I already confirmed that I was posting for him when he was little. And yes, I still help him post. Not sure why that's a problem.
If your primary issue is that his posts are too short, could the solution that I proposed work for you? That until he's old enough that his posts are a little more in-depth, we only post once a day at most, and edit the post to add new stuff if he's got more to add later?
Edited to add: and how do you feel about if we posted pictures of his artwork using this account?