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If you know the correct amount to enter, (the earnings on the 529 account) you can enter the amount of earnings obtained that way, however a phone call is not something you can document and keep with your tax files. Something tangible would be needed to substantiate the entry if it were questioned (even though it is INCLUDING earnings for the CA return, not reporting for a credit or deduction). 

Try to get a copy of a statement from your online account showing the earnings. 

If you decide to rely on the phone conversation and it ends up the amount differs, it won't matter on the Federal return, but your son would then need to amend the California state return. 

FOR THE CALIFORNIA RETURN enter the taxable earnings under the adjustment screen in the California state return.

The adjustment will be asked for when going through the California state return.

You can go to the "Here's the income California Handles differently" screen

Under "Miscellaneous adjustments" you will find "Expanded use of 529 funds" START

Enter only the amount of taxable earnings.

[Edited 02/09/2025 I 9:54am PST]