Try simply reviewing the game. Plays in the first inning matter as much as with 2 outs in the 7th.
Everyone has to do their job so their teammates don't have to pick them up. Create a culture that when mistakes occur and someone does pick up a teammate that it gets called out right there in the game. Example: with 1 out runner on third the batter K's. Next guy pokes one up the middle - the whole team should be on the top step yelling his name with the "Hey Hey Johnny that's a Nice Pick up - attaboy". You should be in the 3rd base box leading the effort. Lot's of finger pointing, fist pumping and that kind of thing. All positive stuff. Make it unacceptable to be negative. For this to stick though you cannot be going against that grain. Not Pollyanna or anything but not a screamer/cusser type. Think John Wooden.
After games in the team huddle things should be called out in terms of Johnny got a big 2 out hit in the 4th rather than Sammy hosed up not getting the one out RBI in the 4th and Johnny picked him up. Both guys get the message you want. Every 17 year old wants to be told they are good....by their teammates and maybe more than by the coach.
Your guy believes he won't get the ultimate penalty which is benching because you need him. If that is actually true and he is a SR. you've lost this round. I would ask you though, if you lose with him, what is the cost of losing without him? Especially if he gets the point. Even if he doesn't and everyone understands what you did the only people you will hear from are the boys parents.
I'd also get with the Freshman and JV players & coaches and change the culture of the program so they don't know anything different. Work on the best players so they become the leaders of the new culture. Meet with them 1-1 and stress that you are looking for them to be the new leaders and tell them what you want. For this years crew do what you can do to set the new pattern so that when the next set comes they know what they will be getting.
Consistency on approach matters. All the best HS programs I have seen you can't tell the JV from the Varsity based on their approach to the game. They get off the bus, warm up, play and conduct themselves the exact same way. Sort of like the old "Oriole Way". It is just understood.