I will echo what has been said above b/c it applies to cadets/mids at all SAs. Many, many folks who largely breezed through h.s. find the SA academics to be a whole new world. Myself included.
Here are a few suggestions:
Doing the same thing the same way for another semester is unlikely to product different results. It may have worked in h.s., but isn't working here. So, something has to change.
Be willing to accept help. Do not be too proud or too stubborn to say you can do it on your own. Just saying, "I'm going to study harder" is unlikely to succeed unless you currently aren't studying (not the case for most cadets).
I've no doubt that USAFA has a variety of programs designed to help cadets. These can include time management, study skills, specific course tutoring, etc. Take advantage of them.
Consider voluntary summer school (I assume it's offered at USAFA). This can allow you to get ahead on your courses and reduce your courseload (number of hours per semester) during Ac Yr.
Determine whether you are trying to do too much. For example, if you're doing varsity or club sports, choir, band, or some other activity that takes away from study time, you might want to d/c it for some time. This is NOT to say that you should only study -- everyone needs activities to de-stress. However, if the ECA/sport is gobbling up your time, you may need to put it on hiatus.
In my day, the SAs were somewhat indifferent to your academic struggles. Today (at least for USNA), there is much more attention being paid to ensure cadets are academically successful. Which is a good thing.
Good luck to your DS!