Baby sitting my uncle's dogs, one is scared what to do?

You probally have no relationship with the dog... as far as i understand he in a pen or something. I would sit outside the pen for a LONG time! talk to the dog. see its reaction. every few minutes get closer to the pen and see if he gets angrier. if he does back up and show him respect. give him some meat! dogs cant resist meat! also mastiffs are very protective for their families! give the dog some toys. once he gets used to you and the time comes... take a chance and let him out! just incase i would find a goodhidin the dog cant get to incase of a bad reaction. call your uncle.