The circumstances of your case suggest that holding a byelection is illegal. This seat is reserved for the person of scheduled caste. Other backward candidates cannot contest from this seat. The district election commissioner has cancelled the candidature of the winning candidate only after the completion of election.
I think that the district election commissioner would have issued the certificate in his favour. Hence, the entire election procedure has been completed without any illegality. His caste certificate shows that he is an OBC candidate, this evidence is sufficient to cancel his election. He has no ground to challenge the election procedure. Therefore, the election commission should declare the runner up candidate as the winner from this seat. The election commission cannot hold byelection in this scenario [Pramod Laxman Gudadhe vs Election Commission of India AIR 2018 SC 2356]
File a writ petition under Article 226
In this situation you move a writ petition under Article 226 of the Constitution of India. The High Court may issue a writ of mandamus thereby directing the election commission to declare the runner up as the winner.
Because there is no procedural defect in holding the entire election. No candidate has any grievance towards the manner of holding the election of gram pradhan. It proves that the election is fair and conducted in accordance with the law. These facts will stop the election commission to hold byelection [D. Sanjeevayya Vs. The Election Tribunal of Andhra Pradesh AIR 1967 SC 1211]
Your claim is genuine and you’ll get relief from the High Court. So you should file a writ petition before the High Court under Article 226.