I have done returns for my son (not a dependent) for years. He now wishes to do his own returns. How can I transfer his data without him having to reenter all data?

If your son wants to use his own account, then unfortunately, returns can't be transferred from one account to another. However, you can still give him PDFs of his past returns for his reference. If he has a CRA MyAccount login, then he can use Auto-Fill, which will not only enter his slips for him, but also some carry-forwards, like tuition.