And keep in mind, as parents receiving letters that you are going through ups and downs with them, delayed and filtered.
Delayed because mail isn't immediate.
Filtered because your kid is going to talk about what he wants you to know. On a hard day it may be able how much he hates it. On a good day it may be able how patriotic he feels. If it's a good day, and then right before he writes something discouraging happens, he may focus on that, forgetting to pass on the good parts.
It's up and down, for a summer and then a year.
I had a chance to read my letters to my parents, last year.... so that is about 11 years removed. Boy was I all over the place. Some days I said I was doing great. Some days I was really down (and I know I didn't write how down I was sometimes).
What you can be confident is, feeling bad and good, up and down, confident and freaked out..... they're all normal. Each day will feel long for them, but the weeks seem to fly by... and while their perception of time is all over the place, for you, it will feel like an eternity.
The best thing you can do is encourage them, remind them that, no matter what, you will always love them, and (when they do call) let them vent.... just let them tell you whatever they want.... and then encourage them some more, and remind them of your love, NOT MATTER WHAT HAPPENS. Unconditional love is less pressure than pride. "We're proud of you" is great, until the kid things he'll let you down. Love your kid, sink or swim, reminds him or her that there is something bigger than this moment in time (something that can be very helpful to remember in stressful times).
Those are my opinions... now 12 years removed from Swab Summer, TRYING to put myself back in my old ugly running shoes and smell gym cloths in New London, CT.