Its a tricky one: He's right in that its pretty hard to "turn off" your feeling if you like someone...it just happens to you and sometimes you cant help feeling that way. It's not really our feeling that we have controll over.
What we DO have controll over is how we act upon them: does he flirt with her? Does he make you feel loved and like you are the one he likes better? Does he spend enough time with you/treat you ok generally?
I'd love to say that if you are in a good, supportive, loving relationship one of you having a crush on someone else isnt a problem but of COURSE it's going to hurt finding the one you like likes someone else as well - it feels like a betrayal. Im a reasonably jealous person, so I sympathise, but what I would do is accept he cant help fancying her, and talk between you about what is acceptable behaviour. Maybe you dont want him to hug her, or to say flirty things to her?
I've also come from the other side of things and been in a relationship where I not only had a crush on someone else but was in love with them. Believe me, if he's a decent human being he wont be feeling smart, clever, egotistical or thinking of trying to get the two of you in a threesome, he'll be feeling guilty, annoyed with himself, and worried about hurting you.
Strike a balance between resonable concern and paranoia about this and Im sure he'll understand.