It sounds like he's not incompetent so much as he isn't efficient. If he were better at prioritizing, would he be contributing value?
I'd first try to get the group to condense the 10,000-feet view into a single document, including priorities, and get the group to agree on it (presenting it as a 'helpful reference').
Then you'd have an easier time saying, "Hey Bill, whatcha working on?" "I'm really bogged down with <task>." "Huh, is <task> really that important based on the priorities in <document>?"
If "no": "Awesome, slash it and move on, dude. What's your next task?"
If "yes": "Huh. I'll talk with some of the others, see if they agree or if you can drop this and move on to something easier."
I'd first try to get the group to condense the 10,000-feet view into a single document, including priorities, and get the group to agree on it (presenting it as a 'helpful reference').
Then you'd have an easier time saying, "Hey Bill, whatcha working on?" "I'm really bogged down with <task>." "Huh, is <task> really that important based on the priorities in <document>?"
If "no": "Awesome, slash it and move on, dude. What's your next task?"
If "yes": "Huh. I'll talk with some of the others, see if they agree or if you can drop this and move on to something easier."