I was able to get an earlier appointment with the vet. In fact, I just came back from his office a few minutes ago. He gave me good news - there isn't anything serious with my baby (Calhoun). He's developing really well and his body/shell are strong. If anything, it might just be a slight respiratory infection (like you said). The vet prescribed some antibiotic drops to give Calhoun twice a day for about a week or so and suggested I try a different kind of food to entice him to start eating again.
And even better, the vet gave me the okay to put Calhoun back into his larger tank. Which is good, because the other turtle (Camille) seemed a little sad that her friend was missing.
Anyway, thank you for your advice and your kind words! It really helped, considering I was a wreck over my poor Calhoun. 
