My boyfriend complains so much. It’s in a fussy almost child-like sort of way. I swear he wasn’t like this before and this is a new “thing” that came

Ohhh I deal with this a bit too. I dunno some people are fussy. He's gotten comfortable with you and he's no longer on his best behavior.
Tell him. Dont nag him about it but just tell him to not sweat the small stuff so much because you prefer a less stressed out life.
Tell him that he can only complain once a day so choose wisely.
If he's complaining more than once daily maybe the relationship isn't working out.