Well, he could be better for his age. BUT, he's still young and there's time to improve. There are many 6-year-olds around the world who are swimming a lot faster, and with more efficient strokes.
He's still a baby. Let him enjoy his swimming. and post another video in about 3 year's time. We are bound to see a vast improvement.
You were given very sound advice when you posted this same video on a UK swim forum.