Sorry this is late. I've been a bit ill and I'm trying to get my Senior Reactor License at my facility.
1: Thank you and thank you to your sons for their service. I admire you for raising two boys under what must have been trying circumstances at best.
2: Keep on him, the rewards of making it as a nuke MORE than pay for any extra hours one had to put in. How we act when most challenged is what helps define us.
3: I never had problems in either A or nuke school or any other nuke training. At nuke school I think I logged about 40 extra hours total. However things I've seen work: Get a buddy who understands concepts and have him explain it to you. This works because sometimes fellow students have good physical examples. Here's a story, I've always been good at Reactor Theory, in fact in 20 some years as a nuke I've never missed a Reactor Theory question. When I was getting my Reactor Operator License at my old utility this instructor came in and explained Doppler and Resonance Capture concepts I understand quite well. He provided an explanation that I use until this day. I'm a mechanic by my Navy Training yet I understand Electrical theory very well because in my mind I reduce generators and motors to groups of Rubber Bands and how they react when under tension. I've used this explanation and in fact had an Electrical Engineer tell me once had she heard how I explain Motor and Generator theory when she was in College I would have saved her 2 years of confusion and at least two courses

My point is sometimes sometimes SOMEONE has a piece of info that will REALLY help your son, and I'd place money on it being a peer. He might also find they're just dying to help. Out of the hours I spent in "study" in Nuke school most were spent tutoring. He needs to make sure he gets someone he trusts though.
4: I've seen this but cannot guarantee it'll happen. Lots of times there will be a point in Nuke school where the trainee has picked up enough knowledge that he/she isn't aware that they've even absorbed, then it just CLICKS. It happened to a guy named Werle in my nuke school class. We called him Whirly Bird because of his last name and at times he was rather a space case. I think he carried about a 2.7 or so and was borderline on many of the courses. About the time we got taught everything it just clicked for him. The totally smoked the comps I think his lowest was a 3.9. So there is hope. I’ve seen guys in the commercial world be the same way, they’re about average until all of a sudden they see how it all integrates.
5: Tell him remember this, 2.5 gets you into prototype as a nuke and from there he should do fine. Again another guy in my nuke school class. The Anchorman ie the person who all who are after him failed. IIRC the school even gives an award for it. Ours was Robbie Schneider. He got a 2.5 even, I dount he ever got higher than a 3.0 on any test but I bet if you ask anyone in 8502 MM “F” They’ll say they respected and admired Robbie more than ANYONE on the class. I believe Robbie averaged something like 65 hours study and after the Section Leader realized how dedicated Robbie was he took him off Mando. They in fact finally had to order him to slow down a bit (This was when Robbie was putting in over 70 hours extra). One thing I remember about Robbie, he’d use every resource, instructors, other students or go to the Officer side of the school. If he heard an instructor explained a concept well Robbie would find him or her regardless of whether that person taught my section or not. He’d recopy his notes because he felt in doing so it would burn into his memory. He white boarded, reread the texts, asked for extra problems. When we got our scores the guy openly cried because he was so happy he passed. My understanding is when he got to prototype he tore it up and qualified in some ungodly fast time. A guy I know said Robbie did well in the fleet.
So there is hope for your son. A LOT of good advice was given here by a lot of good people who survived and thrived in the program. My best advice, when he finds something that works he should just stick with it and not try to find something that works “better” What matters is does it work for him.
Mike