Send him, remember the curriculum isn't like they are at school they aren't sat at a table all day listening to the teacher. They learn through play and you'll probably find he will help his development not go backwards
Going backwards isn’t really a thing and there’s definitely more to gain from going to nursery than just the planned for curriculum opportunities. It’s going to be good prep for school- there will undoubtedly be parts of the EYFS curriculum he knows at school when he joins, and there it’s going to be taught anyway with the expectation he listens with the other children, and demonstrates his knowledge and skills against the curriculum, as well as managing his boredom and self regulate his behaviour. All good practice!
I felt a bit similar before my son began, not that he’s super forward or anything but I don’t really think he’s learned anything new since he started a few months ago like everyone told me he would! It’s more about socialising & getting used to that sort of environment
I'd still send him for the social side of things