Q: I don't have a credit card to create my sons Apple ID

Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method - Apple Suppor…

Accepted forms of payment vary by country. You do need some valid form of payment in order to set up the ID for your child. Debit cards will work in the U.S.

Set up Family Sharing - Apple Support

Where are you located?

A debit card can not be used to set up an account for a child under 13 under US law.

I stand corrected.

There is a link in the article that I posted above takes you to this site where it is explained in more detail. I realize that you know this, but this is posted for the benefit of the OP.

Family Sharing and Apple ID for your child - Apple Support

Create an Apple ID for your child

Before you begin, make sure that you're using a credit card or UnionPay as your iTunes Store and App Store payment method. To comply with child online privacy protection laws, you will use the CVV, verification code sent via SMS, or security code from a valid payment method as part of providing your parental consent. If the card on file is a debit card or another payment method, you’ll be asked to provide a valid payment method before you can continue. After you create the child's Apple ID, you can change your payment method.