I've never let my children down but I feel my boy has really let me down with false hope once again.
He wants to be with his mom - maybe get one on one attention from her rather than spending every weekend with his dad's girlfriends kids, and his two other siblings. Some kids go through periods of wanting to be one on one instead of getting lost in the shuffle of 5. I don't think you should force him to stay overnight. Maybe its best to some weekends do things one one one with ONE of your children, and then the OTHER - away from girlfriend, her kids, and the other siblings. Also, have you ever done something with your two kids and the new baby to solidify their relationship as siblings?
Also, why not move closer to your kids? Be a part of their daily life instead of shuttling them around?
Maybe mom is manipulating him, but maybe its time to go to family counseling instead, or going one on one to visit your child?