The cardiologist appointment went great. It was a bit crazy getting to the doc's office. It was much further than I thought and we arrived a bit late. But the doc was running quite late, so it all worked out. The doctor looked at his file, re-did two tests (an ECG and an ultrasound) and said that everything's fine and my son can get back on the other medication, which he really needs for a much more pressing situation.
Now we're back where we started about three weeks ago--getting him back on the medication, finding the right dose and finally seeing some positive results. The situation is far from solved but we're back on the right track.
Thanks for all your good wishes!