Is Mendelssohn so under rated? I don't think he has virtuosity and freedom like Chopin, or tormented music like Brahms, but my god, Mendelssohn somehow always seems to conjure the most pure beautiful melodies!!!
I think your son's playing is beautiful, or this song without words is beautiful, or both. I can't tell. The music is getting through 100% to the soul so that's all that matters

Are you concerned with wrong notes or your son?
I won't blow smoke, I hate wrong notes like poison. But at this age should this be the main area of concentration or should technique building or something else.
Human beings have a tendency to desensitize. To drugs, for example, we don't feel the effect after a while and need more. So probably same with performance. Just do a ton of them and he will not get as nervous. But I wouldn't bring this up to him too much. It could take the fun out of things if you are too self critical, especially at this age no?