I wouldn't feel bad.
From your description he didn't sound particularly open to the powers of Reiki or the deeper levels that you could achieve.
It must not have been meant for him to hear from you that he was misdiagnosed. It doesn't sound that he would be open to such a suggestion anyway. For him, medical advice comes from doctor's only.
But I agree with what was said about recommending a second opinion if you feel that he need one. So maybe that was the lesson for you.
