Hi and welcome. I'm a Licensed Master Social Worker with more than 20 years experience working with individuals and families on a variety of issues.
These situations are extremely frustrating and can cause a lot of stress but this guy is a grown man..bad manners and all and you get to decide when and how to deal with him. Changing him at this point seems unlikely.
so while it frustrates you and causes harm between you and your partner I believe the best thing you can do is put your boundary down when you need to and let other things go when you know there will be no benefit as he will not respond.
Clearly there are long standing relationship issues here between he and his Dad and that space between them can only get worked out between them so if your partner decides to do nothing about it then you will be spinning your wheels and banging your head on the wall.
you have been wonderful all these years to him and dont respond with the same rudeness but you can have a boundary for what works for you.
"pushing" your partner to see all this with his son most likely will be a losing battle for you...he has his own feelings around it all and deals with it how it works for him. so the less strife between the two of you around this the better.
Seems like you are unable to see this chat. I will switch to Q and A forum