So... you want to keep him? Okay, this is a lot of work, Google will be your friend, as well as searching www.LuckyMojo.com for spells. I also have a lot of how-tos on my old blog: http://ldygry.livejournal.com
First, get her out of the picture. Do a break-up spell on the two of them. I like moving candle spells. You can also look up a lemon break-up curse. You can also hot foot her to get her physically away. There are many options, but as long as she's there, she posed a major obstacle.
After starting that work, then you'll need to strengthen your relationship with him. I like long-standing candle work for this - think of erecting a permanent space for which you'll do love work on him, because if you want to keep a man like this, you'll have to do work on him constantly. Try a honey jar spell. Lodestone magic is also fantastic for keeping a relationship strong. Feed him your menstrual blood every month. Google "nation sack" and instructions to "tie his nature."
Obviously, this is highly coercive work. I don't believe that magic is some special woo-woo energy that returns back and creates repercussions. People do hateful and highly coercive things to each other every day and then for some reason get all hyper ethical when it comes to magic.
That being said, i don't necessarily condone doing spellwork like this lightly. You are seeking to affect the natural order of the world - AND people do that all the time through strong words, threats, guilt trips, fear, reward, expectation, and punishment, etc. Magic is energy just the same as intention, thought, and action - there's little difference.
I am simply of the belief that love should not be that hard to warrant six hours a week of spellwork - your mileage may vary and people have been using this kind of spellwork since the beginning of time. It's just a lot of work - a lot of work to do and a lot of work to maintain. It's not just a one-stop and you're done, a 30-minute-special. You are feeding an energetic link of your creation, and that takes time and effort.
Even though you two aren't formally broken-up, this article i wrote several years ago has some good advice for people who seek to reconcile their relationships. I wouldn't rely on magic alone to do this job for you: http://ldygry.livejournal.com/6296.html
Best of luck!