Simply put, you cannot say anything to him. Actions speak for themselves. The old saying, "actions speak louder than words," is an absolutely truthful one. Trust is earned through actions that suggest a person can and should be trusted, and is therefore, trustworthy.
There is a saying in business that it takes years to build a loyal customer, and seconds to lose one. Trust operates much the same way. You will have to build trust. Once its built, its also a fragile thing. Break it and it is lost forever.