He is not being jealous by calling him a con-man, thats what he is, not only is bad teaching methods for singers a good way to screw your vocal cords by the time your 35 and your not getting what you pay for.
On the other hand these students might be younger and want a "quick" fix..they don't know what actually goes into singing, they probably want to be the next "pop star" and he gives them what they want, they don't want to learn how to properly warm up or sing correctly they just want to sing.
You probably teach correctly and if your a good vocal coach your also an accompanist (sp) and good vocal coaches can be quite boring.. I don't think it has anything to do with being black or white in this situation, I think its the people getting trained, ignorant to music.
It can also be the person, can you give them what they want without being boring.
I have 2 vocal coaches, they both have degrees in music and teach for a living, it wouldnt matter if they were black or white to me what matters is, can they hold my attention, do they make me feel comfortable, do they teach me so I can learn, do I sound better when I go home.
Also not being trained he might be cheaper, what are your rates? What are his?
I pay 50$ for an hour lesson with one coach and 35$ an Hour with the other, they are both just as good.. of course I go with the cheaper more often because its singing lessons and I don't want to spend to much money on something I do for the joy of it.
I think hes just bringing something to the table your not, and its not his skin color..Good luck to you.