Honesty is always the easiest way to go, how else will he know what went wrong and maybe be able to fix it for you, or at least fix it in his life in general. And like a previous post said, you haven't been dating this man very long - if you've decided you're definitely breaking up with him, you don't owe him too much and its better to get out now than drag it out. It's impossible not to hurt someone when ending a relationship, but dragging it out hurts even more.
Plus, i don't know if your daughter has met him yet, but you don't want your daughter to get too attached to him only to have him disappear shortly down the road. That's why i'm a big fan of waiting to introduce a new significant other to a child (or at least just introducing him as only "a friend") until after the relationship becomes a little more serious and you're pretty sure they're going to stay around a while. It can be too much of a rollercoaster for the child.
Judging by your previous posts you are uncomfortable with some of his "hobbies" and trust me, don't expect them to change. Sometimes people can change if they want to enough, but you can never count on them to.
Good luck and hope it all works out for you whatever you choose!