Is it permissible for a man to wear a string and a medallion with the word of Allah (Allah's exalted name) and the name of the Prophet, peace and blessings be upon him, around his neck?

There is no harm in wearing something upon which the name of Allah the Almighty is written. However, the noble name should be protected from disrespect, so it should not be placed in an impure place, nor should one enter the lavatory with it.