I agree with lravidlearner, who reminds that kids are copiers, especially little ones.
 
Do follow her counsel re not exposing you son to this language either on TV or in the house.  If there are older siblings, they will need to be brought into the equation. the parents will have to be very diligent with their language.  It is easy for "bad words" to slip out during frustration or stress.
 
Not giving undo attention is a good idea, but how about coupling that technique with its counterpoint. In other words, consider giving special attention (even 5 minutes can be effective) for using appropriate words.  I might change the "good/bad" idea to "appropriate/inappropriate."  Also, you can try talking about words that "work."  Words that work are those words that make everyone feel good and get positive attention for the child.  It is important that the inappropriate words not get the child what he wants. In other words, when your little one uses the bad words to get his way, do not give in. Suggest he use appropriate words, and when he does, do your best to fill the request when it is safe and reasonable to do so.
 
One other suggestion.  You might want to sit down and speak with the day care teacher.  It could be that your son is actually hearing these words there, however, I would refrain from pointing that out. Putting a 2 and a half yr. old out of day care for using bad words seems like a very strong reaction.  Perhaps you can ally with the day care director/teacher and enlist their assistance in a behavior "plan" for supporting your son's use of words that work.
 
Give it some time.  If you are consistent in supporting the words that work, keeping your son from hearing these words as much as possible, and finding a way to bring the daycare into alliance,there is every reason to believe that his language will improve.
Bette J. Freedson, LICSW, LCSW, CGP
Clinical Social Worker
JustAsk Expert