My sympathies. When I was a USNA Battalion Officer, Chem and Calc were and are still the "plebe-killers." It is indeed a shock that for all these high school high performers, the realization that someone at their Service Academy gets the C, D or F grades. And it could be them.
Express loving support and confidence from afar. Remind him that the SA had enough confidence in his skills to choose him over thousands of others. He will either figure it out or he won't, especially the time management part. I am sure USCGA is like USNA, with a culture of offering EI, company study groups, or classes in effective study skills and time management. His priorities will shrink to water, food, sleep, PT, study, swab duties, sport. He will have to ration social media and goof-off time, and as I noted, he will either figure this out or not. If he was the kid in HS who could read the material for the first time in the AM before school and ace the test, without organized studying and effort, this experience can be a very cold shower. It is also agonizingly embarrassing to feel stupid after years and years of success.
If he is an introvert by nature, it can be hard during first year at an SA to get the private re-charge time he needs to thrive. I used to find mids tucked away in all kinds of unusual places around USNA, desperate for some alone time.
If he gets himself through this, the self-confidence and pride will just flow from him, and he will climb out of the hole. One of our sponsor mids said "my brains grew back" her 3/c year, after a horrendous plebe year, and she had figured out the routine that worked for her. She ended up on the Supe's list and graduated in the top 100 of her USNA class. She is now an extremely successful naval officer headed for command, who counts that plebe experience as something that helped build a leadership style that understands struggle.
DS should evaluate if there is anything he can take off his plate that does not fall into the water, food, etc., list above.
If this does not work out, take it as a "meant to be." If he still wants to be a CG officer, there are some other paths. If he no longer wants to serve, then he has learned something valuable about himself, that perhaps this wasn't about serving but about competing to get into an SA. All those lessons about resiliency and self-reliance you instilled in him will bear fruit, and you will have to sit back and let him find his new path. His brains will grow back, and he will find a way to succeed. I still remember one mid, academically separated after plebe year, a great kid, whose dad, uncle, granddad and great granddad were all USNA grads. He failed, and he was horrified by what he saw as letting down his family. He got into Georgetown, excelled, and is now a highly successful career Foreign Service officer. His family couldn't be prouder of the service niche he found for himself.
Take it day by day, express confidence, encourage balance, and say you know he will figure it out, whatever way it goes.