Hon ~ I can empathize .... the good bye ritual is something that is important to me as well ~ I like to end the day acknowledging the children for something they did awesome and appreciated when parents and children reciprocate that role modeling of courtesy and respect .... sadly it sounds like the family does not share the same values as you and well we can only do so much with children when we have them cause well their parent is still the 'major influence' even when we often spend more time with them when you take into account 'awake hours over sleeping hours' in a week!
Personally if the parents are not on board with raising their child to be respectful and to say thank you or good bye as part of the departure ritual it is going to constantly be a battle because when the parent shows up the child is going to start using strategies that work at home or with the parent in question ~ it sounds like you have tried to 'talk' to them about things and they are just not 'getting it' so unless it is something you are willing to draw the line with the clients over I would just stop making a big deal with this child at departure and see what happens ~ who knows if he sees that he has lost the control of making you upset by not saying goodbye and he is not getting dad or mom to 'laugh' about his rude behaviour he might come around on his own and start saying it on his own!