That's perfectly fine, he should not convert if minor tips are an obstacle. Conversion is possible if someone really wants to, but is not really encouraged (and perhaps even intentionally discouraged, different branches of Judaism have different attitudes towards this) - in stark contrast to Christianity and Islam, proselityzing and bringing every soul worldwide to God is not considered an important goal.
Judaism is for the Jewish people, and while some individuals may join (e.g. in marriage) the Jewish people, from the perspective of Judaism it's perfectly okay if all the non-Jewish peoples (i.e. most people in the world) stay non-Jewish forever and their relationship with God is different.
That sounds like a bad incentive. If a shabbos goy decided to convert he would have to stop being paid, which means that you would be paying the shabbos goy to not convert.