Sudden defiance in 6 year old.

Hello I.M's!
I'm posting in regard to my little boy! My almost 6 year old has over the last two months turned into a complete (no other word for it) turd! Complete defiance over everything, and endless back chatting all out of nowhere! He is a relentless whiner, though has been from day one so nothing new there and managed to cope fine until recently when every single thing is an argument or a smart remark! It's all a power struggle and I'm losing my mind trying to figure out ways to manage!
He has had a rough trot, he has a language disorder, as well as borderline cognitive delay, so confidence is low and getting explanations as to why he is being or feeling this way from him is impossible. I'm feeling terrible but I'm counting down the days until he starts kindy in some hope it will turn things around because he is honestly making life miserable!
He is in school one day a week, at home with me the rest, have cut back screen time in order to take on more one on one time with him and I'm praising him endlessly over every minuscule tasks, I just don't know what more I can do! With a 3 year old and another on the way (which I don't feel is the problem, he talks excitedly of the new baby) I'm on the verge of a mental breakdown! Please give me some guidance!