My son lives with my mother but wants to live with me? she doesn't agree! what can i do?

Right i shall start at the beginning and give you the full picture and hopefully there is someone out there that can help me! I was 15 when i had my son (i am now nearly 26), i didn't cope very well at being a mother that young and suffered post natal depression almost instantly (i admit i wasn't the best person to care for him), so when my son was 8 month old (after a short time spent in foster care) i was told by social workers that i either agreed for him to live with my mother or he would be adopted, i was young and didn't understand the legal system so i agreed and off he went! i maintained regular contact as much as i could! I was in a destructive relationship with my sons father (violent and abusive) and nearly 2 years on i had my second son (full brother to my 1st born)! not long after he was born i met my now husband and found the courage to leave my sons father. After a few years living near my ex we decided to move away where; as a family; we are totally settled and happy, steady job,steady home etc. Anyway, on to the present- over the last few years my eldest (still living with my mother) has been going through some problems and expressing his want to move in with me,my mother doesn't agree, and as she has a residency order, i don't know where i stand! needless to say my mother and i have have discussed this in the past, but nothing has been done and i have also gone down the court route but as soon as i do she cuts me off and i very rarely get to see my son (we live 3 hours from each other), so i give it all up just to be able to see him! The older he gets the more he wants to come and live with me, his step dad and his brother. He is 10 and finds it difficult to tell my mother how he really feels as she resorts to emotional black mail saying things like "don't you love me?" and "ain't i good enough?" which really upsets him so he never mentions it again until he gets in a temper and is so angry that it all comes out! This happened recently and i cant stand by any longer and let my little boy feel unloved and unwanted by us! I would love nothing more than him to live with us but every time i mention this to my mother she finds excuses as to why it wouldn't work! My suggestion was a trial run during school term to gauge whether this is really what he wants, because he has been with my mother so long and he has friends and a life up there, i don't want him to make a mistake and realize before its to late that actually hes quite happy where he is! She maintains that when he is 12 (legal age a court will listen to a child's wishes) that if he still wants to then that would be the perfect time to make the move! i strongly disagree as at 12 he will be entering secondary school which is difficult in itself without any friends, so i suggested a few weeks in a school in our area, doing the normal day to day things without it being a holiday (he comes to us most school holidays) to see if he can cope with the change and if he settles in! I know this was long winded and im sorry but i could really do with some advice, im breaking my heart watching him suffer and it seems that my mother doesn't really want to sort this amicably but i really don't want to go down the court route as it will only make things worse! Please can someone give me some advice, or if there is anyone out there that is going through something similar! thanks x