Does he actually need a 'nice nurturing school'?
Does he need a school that specialises in children with his particular behavioural difficulties?
Does he need a school with very strong structures and discipline?
Does his home environment help or hinder him in modifying his behaviour - in other words, would he do better in a residential setting, or a non-residential one that works in strong partnership with parents / carers?
Does he need a short period at a specific facility - for example a Pupil Referral Unit, or education by tutors who work specifically with excluded pupils - followed by re-integration into mainstream?
Does he need a modified curriculum in mainstream, based on a diagnosis of his specific educational needs? (bearing in mind that SEN can be behavioural, as well as academic)
In your position, I would look very carefully - with the help of an Ed Psych and his previous school(s) - at what he NEEDS (rather than what you might 'like' for him) and then look carefully at how this can be provided.
I knw children with SEN related to behaviour who have thrived in pretty much all the above scenarios ... though oddly enough, none in a 'nice nurturing school'.