I have entered his passport number and other details including D.O.B. into easyjet.com, it says that if the information entered does not match the travelling document, you will not be able to take the flight.
Does this mean that he will be refused to travel because the passwport office made an error?
"You will not be able to take the flight" seems pretty clear to me.
I suspect the 'workaround' needs to be that you enter the passport number and DOB onto the easyjet website precisely as it is specified within the passport itself.
Then get the passport amended upon your return.