As a baby-sitter and at his age, there isn't much you can do. First, he is repeating it because he hears it, so he is exposed to language like that regularly at home. Second, he is probably slightly young to understand differing expectations in different places. A child a little bit older can easily understand what is acceptable in terms of language and behaviour with one person as opposed to another (which is why children will whine with Nana and not with mom, or vice versa), ut probably not one just beginning to talk.
IAre you on good terms with the parents? Maybe you can discuss it with them. You could express your dimay at his language and tell them you don't understand where he might've picked it up (which would likely be a lie), but that you'd like to work with them to help put an end to it. That might give them the hint to watch their language around the child (at least if him using the language bothers them).
I wish I could discuss it with them but they think it's cute when he does it. I guess it just bothers me because I was taught to never cuss in front of family and in public. I just hope when he gets older it'll stop. :)
Sadly, then, he's not only hearing it at home, but it is reinforced there as well. And it will not stop when he gets older until there is some external punishment for it - like school. And he likely will not understand why he's being punished in Pre-K or Kindergarten because he's grown up being taught it was cute. And even when he learns to control his language in school, he will still likely cuss elsewhere unless he makes a conscious decision to change. It is difficult (though not impossible) to overcome the language habits we grow up with.
^ In a household where you know what the child is hearing, ignoring it will usually solve the problem. Zero acknowledgement of something often discourages the behaviour. Unfortunately in this situation I don't think that will stop it because he is hearing it and receiving positive reinforcement at home. However, there isn't much else you can do at this point, either.