Cash as a joint asset

A man in his 70s has, for his own personal reasons, taken $50,000 from his bank accounts and has placed it in his safe deposit box. He wants his son to have access to the money (or what is left of it) when he is gone, and to divide it between himself and his two brothers.

Thoughts and discussions:

• If his son is not listed as a person authorized to access the box, he may not be allowed by the bank to open it after the father dies, even if the son is given a key. The son should be added as an authorized user.
• Even with the box accessible to the son, the money is still the father's asset and a probate estate will have to be opened. But a joint tenancy can be created for any property, not just money in bank accounts or brokerage accounts. Recommendation: a document, signed by the father, stating that the money is jointly owned between himself and his son, and will pass to the son as his survivor on his death.
• The intent is that the son divide the money with his two brothers. That is essentially an oral trust. Dad intends that Son will get the money for the purpose of making the division. This can be effective, but it may not be enforceable. A written trust is better for the very reason that it makes clear that a trust is being established and what the trustee is directed to do. If Son decides that he wants to keep the money for himself, it may be difficult for his brothers to prove that Dad intended otherwise, if the trust is only oral. The written trust is much more enforceable than an oral trust. 

A note: The scenario is a hypothetical, based on a discussion that I had with another lawyer. It is a rule that actual clients' cases are not discussed at this site or in any other public forum.