I don't love him. I feel like I want anyone else but him. Is it fine if I tell you?
You are kidding! I hope you are. Why do you feel that way?
Another man is a better fit for me.
You should always be honest in your relationships.  If things aren't working out between you and your partner, just let him know as gently as you can.  You will both probably be happier with someone else.
Should I tell my boyfriend all this?
Yes you should but like I said be gentle. Maybe imagine what you would like to hear if it were the other way around. What would comfort you in the least if someone said it to you, that sort of thing is letting someone down gentle-like.