We are in a similar spot. DS6's teachers and I have discussed the following:

1. Philosophy enrichment questions - https://www.mtholyoke.edu/omc/kidsphil/stories.html

2. Doing dramatic readings of picture books for the preschoolers. He is a little ham.

3. Wait a couple of months for a few other kids to catch up and then start that little group on an accelerated language arts curriculum. In the meantime, he'll work on other things.

He goes to an unusual school so we sometimes come up with "interesting" solutions.