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Understood your concern.
Here is the solution:
I assume your son has his number which is a part of your family plan right? Hence ask him to create a T-Mo ID associated with his phone number. This T-Mo ID will allow him to see basic account details like account number and the bill due (which I believe is a good habit as he will understand that his parents are spending a lot and he should be responsible ) but he won't be able to make any major changes like change of plan or card information change etc.
Now ask him to use this newly created T-Mo ID to login to DIGITS in his mobile or on his computer. It is completely safe and the account access he will have is the standard access which is way more stripped down access than the primary account holder access you have.
Please let me know if you have any other queries.