Some approaches to consider.
A letter to the priest with the rules:
I am concerned that the liturgical books are not being followed and so am drawing your attention to these rules.
According to the 2004 Instruction Redemptionis Sacramentum: “[104.] The communicant must not be permitted to intinct the host himself in the chalice, …”.
The document has at its conclusion:
“[186.] … This Instruction, prepared by the Congregation for Divine Worship and the Discipline of the Sacraments by mandate of the Supreme Pontiff John Paul II in collaboration with the Congregation for the Doctrine of the Faith, was approved by the same Pontiff on the Solemnity of St. Joseph, 19 March 2004, and he ordered it to be published and to be observed immediately by all concerned.”
For people to receive Communion by intinction the correct method is given in the 2002 General Instruction of the Roman Missal (GIRM):
“287. If Communion from the chalice is carried out by intinction, each communicant, holding a communion-plate under the chin, approaches the priest, who holds a vessel with the sacred particles, a minister standing at his side and holding the chalice. The priest takes a host, dips it partly into the chalice and, showing it, says, Corpus et Sanguis Christi (The Body and Blood of Christ). The communicant responds, Amen, receives the Sacrament in the mouth from the priest, and then withdraws.”
In the 1975 GIRM it is at 246(b):
“The communicants approach, make the proper reverence, stand in front of the priest, and hold the communion plate below their chin. The celebrant dips a particle into the chalice and, showing it, says: The body and blood of Christ. The communicants respond: Amen, receive communion from the priest, and return to their place.”
If you can get a response it will help establish if he is ignorant or deliberately disobedient.
Other approaches:
Talking with the priest
Teaching the parishioners
Discussing it with someone might persuade the priest.
Writing to the bishop.
Formula Tribunal approach for breach of Code of Canon Law, canon 846.
Publicising the problem.
Gathering evidence such as photographs and videos. (It can be useful to help the above approaches and can be used to indicate to the priest that you are serious about the issue).