Over complainer

My boyfriend complains about absolutely everything from him not having the right jocks to wear to bed to my mum not going through with her diet, nothing is good enough for him if dinner is cooked late it's the end of the world, he has something negative to say about everything for example my mum got a new car yesterday and the first thing he says is 'aw they are the worst for fuel, it's got oil marks there it's definitely not brand new' WHO CARES JUST BE HAPPY! Does anyone have any advice on how to react when he does this? How do I tell him his negativity is disgusting and I can't stand it😂