When the young man receives the call to the priesthood, how soon should he state this to his priest / pastor?
This is a personal and individual matter, to be discerned with a good Spiritual Director, as well as a Vocations Director.
Really, as soon as possible because knowing that the young man feels called will enable the priest to support him and assist him in his discernment. I can understand from my own experience that the young man may feel reluctant (out of fear of being pressured for example) but I would certainly encourage anyone in that position to talk about it with their priest as soon as possible. Their priest may also be able to help them identify a suitable spiritual director. I’d also add that vocations directors require a reference from the person’s parish priest as part of the seminary application process.