You can always file a complaint with the probation officer's supervisor. But then you've put his job in jeopardy, and do you think he will give you son an easy or tough time going forward. A probation officer has a lot of discretion to file a motion to revoke on a person that he is supervising, so I don't know if complaining right off the bat is a good idea.
It might be a better approach to call your son's PO and talk to him about this behavior. Tell him that you don't feel that this is an appropriate way for him to speak to you or your son and that you would appreciate it if he showed you both a bit more respect (or acted more professionally). If the foul language continues, then you might have to consider filing a complaint and getting his superiors involved. But I would only do so after carefully thinking about whether or not this is worth the risk he will turn up the heat on your son. Call the attorney that represented your son on his case or another local juvenile attorney to get more specific advice about how to navigate this situation.
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.
Frankly, I would require him acting inappropriately on a couple of occasions, THEN I would talk to him about his behavior and advise him that no more will be tolerated or you will go to the media as well as his supervisor with the recording. Many times these probation officers are very unhappy in their lives. I usually advise clients to grin & bear it, but if he is spilling his venom over onto you then it needs to be stopped because he is probably even worse with the kids whose parents are not directly present. Moreover, he is certainly not setting the example that troubled children need.