My car is over heating after changing the thermostat, purged the line a lot air

What is your question?

My car was a little on the warm side where when the fans come on they stayed on. I changed the thermostat and now my car really over heats. My mechanic says it could be that the water pump is not spinning properly so it is pushing air through the coolant box making it bubble.How do I know if he is correct, I have already paid him to purge the lines which has a large amount of air in it when purging. How do I pin point what it is because a lot of people that own these cars say they have replaced everything and their cars are still over heating. Please someone have the answer