Death benefit to minor

I am hoping someone can provide me with some guidance.  I have a 2006 terminated employee who died in 2007.  The employee had no spouse and left his 401k assets to his 3 year old son (under $2,000).  I have a copy of the letter that the employer wrote to the family of the deceased in 2007.  They received a phone call from the deceased employee's mother who told them to contact the sister, which they did.  There was no response.

We when took over the plan in 2017 we found out that the employee was deceased and the assets had not  been paid out.  we have sent letters with no response and  we are now well past 5 years.  So we now have a plan failure - even though the beneficiary has no idea that he is entitled to the assets?

The beneficiary is still a minor (16) and  I  have located the beneficiary on face book (I did not contact him).  I have contacted the mortuary who provided phone numbers that no longer work.   I have located the sister and sent several e-mails, she does not respond.  I cannot find a financial institution willing to accept the assets as a rollover into an in inherited IRA.  At this time, I have no idea on how to proceed.  Any suggestions/guidance would be greatly appreciated.

I would look to the language in the Plan document regarding distributions to minor beneficiaries. If it is to a person who is a parent or guardian of the minor, then it should be distributed to the parent  or guardian for the benefit of the Beneficiary.  The Plan document controls.