Answer
According to the Hanafi Madhab, the Imaam and Muqtadi (follower) must be performing the same Fardh Salaat in order for the Salaat to be valid.
The Dhuhr Salaat you performed behind the brother who performed Sunnah Salaat is invalid. You should repeat your Dhuhr Salaat.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai