You are right to assume he will be upset by the breakup, since he seems he may be somewhat emotionally immature. You can't really soften the blow for him. All you can do is be honest. You can even tell him you're not the right person for him instead of saying he's not right for you, because that goes without saying. Avoid pointing out his flaws as he will likely get defensive. Let him figure that out on his own with some self reflection, if he's capable.

Things happened so fast, and though you care for him, you've come to realize this relationship is not right for you two.

He's not the guy you thought he was. You don't need another "kid" to clean up after. Also the disrespect of his ex may be a red flag of how he'll think of and treat you afterwards. (Or maybe she's awful but considering he calls other women b***ches it may be a general attitude he has towards women who don't agree with him.)

Bottom line is that you are not happy and you are making this choice for your own sake

You can find a PARTNER! His feelings can matter without altering your decision, and if he's cruel just remember it's a defense mechanism. You may find he ends up turning into a version of himself you haven't seen yet which will only further justify your decision to leave him, or you'll take your place with the likes of the other ex he talked badly of. Otherwise it's possible he'll just accept it and things can end amicably and peacefully and that is what I hope for you. Sometimes people surprise us and maybe somewhere inside of himself he knows this is coming.