to wean him away from his mother, is not a difficult task. The guy is head over heels in love with you. all you need to do is arouse his feelings for you and follow the leader, Ishwari. You could have used all that you have got ( no, no, i am not telling that you should have got physical with him!!).
Simple examples, follow Ishwari.. See what she does..
1. Put Dev's head on your lap.. how many times did you to it? just once that too when he was emotional? caress his hair... just the way Ishwari does.. the guy loves to put his head on a lap.
2. talk to him softly, use words that would arouse him, do not fight with him, go to the extent of calling him beta.. make him feel like a baby.. give him the matronly treatment... like the way Ishwari does.
3. Finally, do something that Ishwari cannot do.. just kiss that guy.. use your lips any which way you can..
see, now you get it? you lost dev, albeit temporarily, because you did not take good training from Ishwari.. bad girl.
anyway, all the best now, better late then never..