While you can’t force an individual priest to say Mass in the extraordinary form, if a stable group requests it, he must accommodate them. If he is not able to facilitate that himself, he should request assistance from the bishop and the office of worship of the diocese.
From Summorum Pontificum:
Art. 5. § 1 In parishes, where there is a stable group of faithful who adhere to the earlier liturgical tradition, the pastor should willingly accept their requests to celebrate the Mass according to the rite of the Roman Missal published in 1962, and ensure that the welfare of these faithful harmonises with the ordinary pastoral care of the parish, under the guidance of the bishop in accordance with canon 392, avoiding discord and favouring the unity of the whole Church.
Art. 7. If a group of lay faithful, as mentioned in art. 5 õ 1, has not obtained satisfaction to their requests from the pastor, they should inform the diocesan bishop. The bishop is strongly requested to satisfy their wishes. If he cannot arrange for such celebration to take place, the matter should be referred to the Pontifical Commission “Ecclesia Dei”.
Art. 8. A bishop who, desirous of satisfying such requests, but who for various reasons is unable to do so, may refer the problem to the Commission “Ecclesia Dei” to obtain counsel and assistance.