No of course the owner can't do this. It is his fault if he incorrectly priced his listing at the time you booked.
Just reply and say you are sorry that he incorrectly priced his listing but he needs to honour the price you booked at (there's a risk he may cancel but then he will be penalised).
Make sure all communications are on the platform and give Airbnb a call too.
When are you travelling by the way @Stewart85 - at the moment Wales is in lockdown and not accepting bookings for short term rentals