What to do if beneficiary of deceased participant is not responding and we have no SSN for the beneficiary?

We have a 401k plan with a participant who died a couple of years ago.  The beneficiaries are the 2 adult children.  The daughter has already received her distribution, but the son will not respond to any contact from us or from his sister.  He did respond when we first contacted him and said he would complete the paperwork, etc.  He never did and now does not respond to mail, calls, etc.  We cannot even do a forced rollover because we have no SSN to provide the platform that holds the assets.  Any suggestions are appreciated.

Thank you.

There isn't much you can do.  Sorry, I know that isn't the answer you were looking for.

You can't really even say this person is lost and forfeit the balance assuming the document allows for that.  The person isn't really lost. 

I have my share of these and they are a pain.  

Knowing neither the plan’s provisions nor this situation’s facts:

Has the remaining beneficiary’s required beginning date arrived?

What does the plan’s governing document provide?

Are there provisions the plan’s administrator should or may presume will later be added or changed in a remedial amendment and will have retroactive effect?

Might the son be an eligible designated beneficiary?

Ask the sister for the info or her help in getting this done.