Judaism not only does not believe in proselytizing but active discourages people from converting. This is done primarily to insure the sincerity of the prospective convert and is based on the Gemara Yevamos 47 and Shulchan Aruch YD 268 which explain the procedure for accepting converts. It is of utmost importance that one is converting for the proper motives of honestly desiring closeness to Hashem and not for ulterior reasons such as to marry a Jewish spouse. A non-Jew can successfully fulfill his purpose in this world and achieve his portion in the world-to-come without converting, but if he becomes a Jew he will be held to much higher standards and will be punished for failing to live up to them. In this context, I think that it can be explained that the Rabbi is more cautious and skeptical than hostile. In effect his intentions are for the prospective convert's own good, that he should be fully aware of what he is getting himself into, because there is no leaving Judaism if it turns out to be too hard.
It is true that a gentile is told to violate the Shabbos even if he is in the process of preparing for conversion. The reason for this is that the Shabbos is a sacred covenant between Hashem and the Jewish People, and until one has formally joined the Jewish People he may not fully participate. The Rambam even writes that a non-Jew who keeps the Shabbos is liable the death penalty. So, while a prospective convert is taught and trained as to the proper way to keep the holy Shabbos once he is accepted as a convert, he is simultaneously told that he may not yet keep all of the Halachos.