Keeping a friendship platonic can be tough. You know his love is sincere and you don't want to hurt his feelings, but you need the freedom to find someone else without guilt. You share so much that it brings you both joy so ending the friendship isn't attractive to either one of you. Here are some tips on maintaining a platonic friendship.
Limit physical affection. Good friends naturally hug and kiss, but it's inappropriate in a platonic relationship unless something tragic has happened, like a death in your family. Respect her feelings for you and don't give her a reason to dwell on your motives from inappropriate physical contact.
Notice how close together you sit or stand. When you're at a party, for example, watch out for love seats or crowded situations. You may think this is going too far, but his subconscious can send up weird messages from careless close contact, like maybe you're finally coming around.
Be nice to his potential partners. Show him you're happy he's getting over you. Invite her to the party you're throwing or occasionally go out to dinner with them. Let him know how suitable she is if you think she's a great person.
Speak kindly of her partners as long as they treat her right. When faced with an abusive boyfriend, talk to other friends so they give her the message and it doesn't come directly from you. They likely will feel the same way about him, so they won't be lying by keeping your name out of it.
Criticize his bad behavior as honestly as you would any friend. Tell him when he's acting like a jerk. Make him understand you wouldn't put up with nasty comments from anyone and your platonic relationship doesn't change that.
Demand he be nice to your boyfriend. This isn't something you need to compromise on. Remind him you've treated his girlfriend(s) with respect and you expect the same deal from him.
Treat her with respect. A platonic relationship doesn't mean you can ask her for favors and expect her to gladly do your bidding. Take her problems as seriously as you would any friend's.