Family Plan 300 day limit
my son needs to move to a different family plan but he got an error that I am not able to join another family plan for 300 days.
iPhone 14 Pro Max
See the following from Set up Family Sharing - Apple Support
Remember, you can only join one family at a time, and you can only switch to a different family group once per year.
This is primarily a user-to-user support forum; other Apple users like you.
If you want to contact Apple Support, see: Contact - Official Apple Support