How old is he and are there any know problems either at home or at School, has he been in the Pack for long?
Try speaking to him and finding out what activities he likes to do (we have Cubs who don't enjoy craft, but love cooking and vice versa for example).
Can you talk to the parents? Does he enjoy Cubs or is he there because his parents want him to be there?
You could try peer pressure, and rewarding good behaviour of the others and any good behaviour by this child. It need only be a verbal recognition of good behaviour. Most children will respond to this.
One other thing you can do is ask the parent to stay and support you.