One of my sons testicles is not in place, only one is. He has just turned two years old. I have been advised to wait, is this the case?

Retractile v Undecen. If the testicle is "retractile" then it can be easily manipulated into the bottom of the scrotum and will stay there a moment or two. This does not require intervention. If the testis cannot be placed into the scrotum on exam by a medical professional, this is an "undecended testis" and may need intervention - surgery. It won't resolve after two yrs by itself.
No. Who advised you to wait? A doctor or a friend? If it is undescended testicle, it should have been operated on already unless there are circumstances that I am not aware of. If it is a friend/relative suggesting, get your child to be seen now.