Need help with 5 year old tantrums and attitude, any suggestions?

My girlfriend has a 5 year old son and he is becoming unbearable, he is continously screaming and becoming violent and treats everyone horribly, his mother is almost continuously crying about it because he has no respect for her at all and has even told her that he will treat her like a dog because she is one, the only person he ever screams for is his gran and whenever they are together I am literally disgusted, she hugs him and holds his face to her breast and whenever he is told off by anyone she hugs him and tells him he's a little angel or gives him sweets, he can use the toilet by himself but will scream at random ocassions wanting someone to wipe him and if you tell him to do it himself he will either scream for an hour or wipe it on the seat on purpose. Me and his mum have tried everything but nothing ever works and our relationship is struggling because of it, please can someone help or give advice?