You say this "will be" his fourth year -- are you referring to his starting this fourth year in the fall? Why? If Cub Scouts breaks for the summer, take advantage of that natural break to end the activity. He doesn't want to be there. He HAS stuck with it for three years, and completed them, despite clearly not wanting to be there. So your point of "we have to finish our commitments" was made every single year, three times over. Will he really gain something by doing another entire year of Cub Scouts when he doesn't want to be there? Isn't that unfair to him and to the rest of the boys in the group who do want to be there?
He does well in individual sports, right? Have him do those. Find short-term (four or six or eight week) classes in those sports. Community centers and recreation centers run by your county or city very likely have these all summer and in the school year too. Team sports is not a requirement for being a child, though in many areas, team sports seems to be expected of all kids. Don't fall into that trap.. Let him do what gives him a sense of accomplishment, not what frustrates him. He likes crafts, so find craft classes - again, short term commitments because of his maturity issues, no year-long classes. Or do your own craft sessions at home and invite his friends who also are interested.
Save the "You have to finish this" fight for schoolwork, where it matters most. With activities outside school, give him the chance to do what interests him and don't push him to do the kinds of group activities you already know frustrate him. As for the other boys - I'm not sure how they come into the picture here at all.