I originally assumed you meant he ditched you for his girl, spends all his time with her and none with you anymore. THAT is pretty typical with the little boys we have running around nowadays.
But your situation is different, if I am hearing it correctly. He basically tries to verbally demean you in front of you and his girl whenever you interact with her? You can also assume that he talks shit about you when it's just him and her.
I think he has low self esteem and is worried that she will like you better. He doesn't feel like he deserves her in some way so he's gonna be hyper sensitive and jealous and all that fun stuff. Good times.
He'll lose her eventually. Talk to him about it. You don't deserve to take that shit. Slap him around if need be