Make sure the child benefit office know your stepson will be living with you permanently. His mum should notify them as she is currently receiving the benefit, but all you need to do is get a form from the post office :o)
You may wish to get a residency order drawn up through the courts, but if it is all amicable it shouldn't really matter..may be wise to be on the safe side..worth looking into!
Take care & good luck ~ I wish my stepdaughter was living with us, it would make life so much easier :o)
xx