Step son with mild CP and behavioural issues

My step son has a mild form of cerebral palsy. Both his legs are affected but mostly on his left side and is undergoing various experiments to have them made as good as possible. Mentally he is fine. He is clever and actually smarter than his sister's (which they'd kill me for saying).
He has behavioural issues which I do not excuse and he has been disciplined fairly for. The trouble is that he never seems to learn from his mistakes and repeats the same behaviours expecting it to turn out differently each time. 
Another problem is that his behaviour is affecting everybody in the house. There are 5 children and so much focus seems to be on what he's doing wrong that the others feel left out and act out. We are fed up punishing him and explaining repeatedly what he's done wrong. It's got to the point that it's noticeable that we're unhappy. He's not happy because of being in trouble and we're not happy as it's all so stressful. A family member said "is it due to his cp?" Which of course we've thought of but I'm sceptical because when there is something in it for him, he will do what he's been asked despite the fact that he knows that misbehaving equals trouble. 
Is there anyone we can speak to? Is it a trip to the GP? Does anyone on here have first hand experience that might offer insight?

Thanks

Blinx