My son and I share an Apple ID and want to split it into two accounrs

You can't split an Apple ID into two accounts.

You need to create a new Apple ID for him and then consider setting up Family Sharing.

If 13 years old or older.

Apple ID - create a new.      

Apple ID – Create         

If under 13 years old, you will need to set up Family Sharing to create an ID.

Family Sharing and Apple ID for your child.      

Family Sharing.         

Family Sharing - Set up.