Hello Anne,
I had night sweats and also a persistent cough, with no other symptoms, and it turned out to be endocarditis. I did not have the fever that frequently accompanies it. I don't wish to be alarmist, but if your son has a known heart issue and has developed night sweats, I would see your doctor ASAP and have some blood tests done to at least rule out endocarditis. If caught early, it can be treated with antibiotics. In my case, it was not diagnosed in time and resulted in my having my valve replaced.
I would recommend having it checked out by your doctor.