My employer agreed to pay me a new salary (higher) and commission (lower) structure in August, I have received the new pay on 2 paychecks so far. Today he advised me that we are going back to my old pay scale (lower salary, higher commission). Can he do this?

As long as the new pay rate does not drop you below the applicable minimum wage, the employer can change your salary on a going-forward basis. What the employer cannot do is pay you less for hours that have already been worked.