Welcome! Thank you for your question.
You would NOT need Letters Testamentary if there is a beneficiary designation on the 501k account. Assets with beneficiary designations pass outside the probate system and directly to the designated beneficiary.
However, with that said, since the named beneficiary is a minor you are going to have to go to the probate court (where you live) and get the court to establish you as either the guardian of your son's estate or request authorization to place the funds in a court designated blocked account. Both of these will last until your son is 18. Here are your options:
Both of these would have to be done through the probate court where you live with your son.
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