Can my teacher change my grade?

you'd have to prove to him through your work that you could get the A. he can only really predict what he thinks your capable of getting. you can try talking to him to see if there is something specific you could do.

You think I still have time to prove to him for the remainder of the year? even though I have 1 month left

it depends on when your school locks down the predicted grades. do you have mocks after summer before predicted grades are set? or is it just your own mock papers your doing. you might still have time but there is no guarantee your teacher will change your grades