A good rule of thumb is always to talk to the pastor first. Don’t come out with guns blazing though.
After a Mass, just casually remark as you shake his hand “Father, I was always taught that self-intinction was not permitted, is this true?” If he confesses that it’s not allowed, then encourage him to reform the practice of the parish. If he feigns ignorance, or say that it’s allowed, just nod and walk away.
If the pastor said that what he was doing is allowed, after a few days write him a letter, explaining that you checked your sources (In this case, the GIRM) and give him the citation explaining the practice. Sign your name, and give your address in case he should reply.
A week or so after he received the letter, if he still is offering Communion by self-intinction, write a polite letter to the bishop, or whoever he delegates the government of liturgy to in the diocese, explaining that you mentioned this to the pastor and nothing has been done. Include copies of any communications from the pastor you might have received in response to your letter.