They're both used and correct.
However, placing the possessive before the noun can be used as a way to emphasise the ownership; making it the main focus of the sentence. Think "That's MY boy".
In cases where the ownership is not supposed to be emphasised, it's generally more common for the possessive to follow the noun than the other way around, but there are plenty of exceptions. Some of them are fixed expressions, others just sound better that way.