Ooo....
Good luck, it's a very tough one and I feel for you.
I would get all the other guys in for a chat and work out a plan of action that sees a couple of you take this guy out on 'practise' rides three evenings a week and a long (4hr) one on the weekend. Get our hero to practise keeping a wheel and take it in turns to pace him back into the bunch when he gets dropped, keep to an agreed average speed on the flat roads, but wait for the whole group to assemble at the top of a climb.
Never leave Hero on a climb - take turns to pace him up. Getting dropped then abandoned is soul destroying and Hero will get so pissed off he'll ruin your weekend because he'll stop trying and end up going much slower than he would if he had company.
I suggest a competition where the most helpful 'domestique' on each day does not have to buy their own drinks that evening. It's up to you if you tell Hero about this or not!
I've another thought. Does Hero appear to be going well for a short while then suddenly become really slow? He may be going off too hard (to keep up with you quick guys), blowing up and then riding in the red zone for the rest of the day. Someone close to me used to do this - we had a horrible time 'till I learned to mentally relax and my pal learned to not worry about being slow and take it easy at the start. Our average speed was lower than I would achieve alone, but we covered the miles and had a nice time.