How can I co-sign for my son, a college student, to help him establish credit and keep him in the Apple ecosystem?

You can make him a co-owner or participant of your own Apple Card. To be a co-owner he needs to be 18 years old or older.

click here ➜ Set up Apple Card Family and add a co-owner and participants - Apple Support

Co-Owners:

• They share full responsibility for the account balance and all payments, even if one account co-owner does not pay.
• They can build credit together equally, which means that account owners and co-owners are each reported in their own name. Credit reporting includes positive and negative payment history, credit utilization, and additional information. Card usage and payment history might impact each account co-owners credit score differently because each individual's credit history will include information that's unique to them.

If you make him a participant, he can also build credit history if he is 18 years old or older.

• If they're 18 years or older, they can opt in to build their credit and be reported as an authorized user on the account.