As recommended above, pick up
Combating Cult Mind Control, and also get
Freedom of Mind by the same author. FoM contains much of the same information about how cults work from CCMC but in a condensed form, and also lays out how to develop a strategy for talking a cult members into freeing themselves. Both books will help you come up with a plan and understand where the weak points are in a cult belief system.
Your dad sounds like he is attracted to being surrounded by seemingly up tone, up stat, pro survival OTs with a shared purpose of saving the planet and improving themselves. That pull can very hard to resist, especially if he's he some "wins" from Scientology and still has a soft spot for his time on staff and the camaraderie he felt with his teammates, even if he likely was underpaid, overworked, and abused.
The nature of how people come to believe in cults also makes them balk at being confronted head on. No one believes they are stupid or foolish enough to join a cult. They are right -- being smart or stupid is not what makes a difference. No one goes out thinking "I want to give over my money and my mind to an authoritarian group that wants to control my thoughts, what I can see and do, and who I can speak to." But cults get you through other means, not by attacking your intelligence, but by abusing how we respond to social pressures and other quirks of human psychology. People don't think hypnosis or mind control could work on them, but they don't really know what those really look like. How would they even know what being mind controlled felt like, especially if it was being done secretly or being called something else like "doing TRs?"
Being too aggressive will likely backfire because people double down on their beliefs when challenged. That's so frustrating because you feel the urgency of preventing this disaster and just want to grab and shake him and yell "This is all bullshit! Can't you see how crazy this is!?" Sadly that won't work as much as I wish it did...
You may try saying you don't want to stop him from doing what he thinks is best, but you just want to make sure he has all the information. Scientology teaches that you should "seek to live with the truth" and it is supposed to make you able to confront whatever you fear and collect data to make objective decisions. It doesn't really do that, but it says it does, and that can get you in the door.
If he's almost at the OT levels, his indoctrination is very high so convincing him it is OK to look at critical material will be very difficult since doing so would ruin his "OT eligibility" and costs him a lot of time and money in ethics handlings and sec checks. Scientology rigs itself so its followers are scared of "entheta" AKA negative information like the true history of LRH or the stories of ex-Scientologists. You are taught the only reason you leave or even have critical thoughts is because you are guilty of crimes, and only insane criminals oppose Scientology. You have to police your own thoughts and automatically assume all critics are by definition evil criminals. This is what kept me in for so long even once I started doubting. The Steve Hassan books we recommended will explain how this is phobia induction (implanted irrational fears about leaving), and it is a hallmark of how cults prevent people from escaping.
Sit down and write out what you know about your dad. What was his life like before he got involved in the cult? What was it that attracted him to it? What got him out the first time? FOM will give you more guidance on what to write down and how to recruit your family and friends to help get your dad out. Here is more info on the approach:
https://freedomofmind.com/strategic-interactive-approach/
Here are some approaches you might take if you think you have enough trust to talk critically about Scientology with your dad:
Would he be willing to watch Going Clear or Leah Remini's show with you?
Would he read a book like
A Piece of Blue Sky that gives an honest but not favorable true history of LRH and Scientology?
What about short YouTube clips about certain troubling aspects of Scientology? (I can help you find these if there are certain topics you know your dad would be interested in.)
Is he interested in the stories of people who really knew LRH? Many of them are on the record telling stories about him and the history of Scientology.
Would he be willing to read books on critical thinking or science that do not directly attack Scientology but may mention it in passing?
Would he learn about other high control groups like Jehovah's Witnesses, Jim Jones, the Moonies, etc.?
Would he be willing to read Debbie Cook's email, and would that mean anything to him? (If you don't know who that is, she ran Flag, the biggest Scientology org, for decades, but left and sent out an email that gave specific examples of how the current church leadership were violating LRH policy.)
If he was willing to do any of these, would he also be willing to talk them over with you? Would you worry his cult identity would kick in and stop him from being able to honestly taking in this information and discussing it openly? Is he in deep enough again that he would feel the need to report that you are being "suppressive" by sharing critical information, and if they pressured him to disconnect from you and his wife, would he do it?
Be careful about going in too harder and too fast because that will trigger all the mental booby traps Scientology puts in place to trap people. Again, the recommended books will explain this better than I can.
The whole situation sucks but I hope you find a way to navigate it successfully! Good luck!