I respect your reply, firstly in answer to some of your questions,
Whilst in our care he was assessed by the mental health team when he was 12 & 14 and the therapists on both occasions said he's slightly depressed! He's also been referred to counselling by school but he refused to go.
As for trying to replace his mother that has never been my intention I have always been there for his children as guidance, his sister is now in university and his other son is now in the army. Since moving out of the home I don't have any contact with him, which I hope you can understand from my situaton.
The parents separated when the son was 4 years old & I have been in a relationship with my husband since he was 5yrs old. Prior to the mother moving 200 miles away we lived within 500mtrs of the mothers home, he came to live with us at 6 with his brother 8 and his sister 11, as the mother stated she wanted to spend time with her new boyfriend! It took my husband and I months with the help of social services to force the mother to have contact with her children on a Saturday/Sunday (we both work full-time). At aged 8 he decided he wanted to live with the mother, which was fine by my husband due to her house being close by or so he thought. As soon as he went to live with the mother, she stopped all contact we spent the next 2/3 years in and out of courts to no avail

So the 1st time my husband saw his son again was when we were woken up at 2am by the police on a week-day with his son. They told us they had been called by the mother to remove him from her house that she couldn't handle him, and that he had threatened her with a knife, at the time his son disputed this. The following week his mother had sold her home and moved 200 miles away with no forwarding address. It became apparent within a week that while he had been residing with his mother for nearly 4 yrs he had been allowed to decide when he went to school his attendance was 60%! Within 3 months of living with us he had been expelled for abusive & threatening behaviour towards teachers and moved to a school with bars and locks, he spent 2yrs here then moved again to another school due to vandalism, violent & threatening behaviour mostly towards females. The last year he only had to attend school 2 hrs a day 4 days a week, all the schools have bent over backwards to accommodate his needs, my husband has spent many, many hours talking to his son and to be honest I believe too soft... He seems to be wanting to be in control of everything, and if things don't go his way he rages, I feel that each day he pushes the bounderies, he's stolen money, he's stolen our possessions. Whenever he is called on his behaviour he either lies, storms out or blames it on everybody else, nothing is ever his fault, eg. last week my husband got him out of bed at 9.30am to go to school to collect his work for the day, he arrived at the school and smashed the classroom up and was arrested, he told the police it was his dads fault because for putting him in a bad mood because he didn't want to go to school!
Now there comes a time when he himself has to take responsibilty for his own actions and not blame others!