It seems from the various answers, that the bike may or may not be too small.
Some people who are taller than the O/P’s brother are riding that size frame.
It’s not difficult to work out the correct size bike if you know what you are doing, so if the shop is a good one, why would they sell him a bike that was too small?
I would call the shop and voice my concerns. They will most likely tell you how they came to fit him on the 52.
Then you can discuss their decision.
But the guy might want to ride the bike further than round the car park before rejecting it for a larger size. See if the shop will allow this, and still replace it if required.