A real shame about Beavers - they do need to be run very well to prevent bullying. Cubs has been a real godsend for DS1, who sounds a lot like your son.
I also wonder whether it's hard to get your son to agree to do anything, as it is with my son - reluctance to try new things.
If it helps, try and get someone to go with him to something, to encourage him.
Ideas: an individual sport - swimming, fencing, karate, kayaking, sailing, horseriding (expensive!), cycling (BMX), tennis.
film-making, art class, musical instrument
Don't stress to much. He might not be much of a "joiner", and he may be tired after school. And he's still quite young. I consider myself happy if my sport-hating son does one activity a week. Rest of the time he's happy at home.
Sorry - have just noticed you are in Sheffield (I was trawling unanswered messages). I'm in London so don't know what's available where you are, but good news you are in a city.