Well, it depends on the bishops' conferences. The bishops' conferences can establish these rules. I’m not sure which rules apply in the place where the person is asking, so I would need to look at the specific rules of that bishop's conference or diocese. However, I believe that in this case, the priest cannot forbid it. Even if there is a rule from the bishops' conference that everyone must stand, I think the Church should allow enough freedom for the faithful to express their devotion to the Blessed Sacrament.
In this case, it is legitimate to kneel. It’s not something extraordinary or exotic, simply kneeling. I believe the Church and the priest should be tolerant and pastoral enough to allow this. To prohibit it would be very anti-pastoral, a rigid and narrow view, which does not help faith or devotion among the people.
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