It's not unknown for the male partner to get residency (my ex's ex partner did) but really the simple answer is see a solicitor. At 12 I daresay you son might have a say in who he lives with too, but don't hold me to that.
There is no reason why you can't have a residence order. Since 2005, if a child is 9 years or older they more or less have the final say over who they choose to live with in cases like this. You need to see a solicitor who specialises in family law. If it goes to court, a CAFCASS officer will be appointed who will interview all parties concerned on a one to one basis (including your son) and he/she will then make their recommendations to the court.
Good luck.
hi i'm 15 now the same thing happened with me when i was 10 but i lived with my mam i was able to have my say so he shuld i choose my mam cause i didn't lyk my father am sure he will pick u
why can't you sort out a jopint custody? Whoever can have him in the schoolweek have him monday to friday and then the other one see him weekends, at least to ease any change over of routine, it is a difficult time to be messing with kids heads and if the mother is doing it or financial reasons that is wrong, if you claim all the allowances for him make an arrangement to share, it makes more sense, it will end up in court and a fight so asct sensible now