Vesting in this area basically means that you have fulfilled all the requisites to be entitled to something. When it comes to benefits, it is usually a time requirement. Most probably the company required a certain length of time employed to become vested, meaning to be entitled to the benefit. You could contact the company and find out what where the requirements for becoming vested and check if your son actually did not meet them.
This does not create an attorney/client relationship. This does not constitue legal advice. It is limited to facts of the question. You should consult an attorney before making any decisions based on this answer.