I truly appreciate the gesture you have shown by being with him and helping him while other's are teasing him. That is like a true friend. Now, i would like you to look at everyone around you in your class keeping in mind your expectations from a boy or a girl in looks, mannerism, voice or other characteristics. You would see that everyone is different and what is in our head is just the model for average. People tend to tease or ridicule when someone is above or below average. Your friend had been a target here. He would need to visit a psychologist/counselor for two things. Firstly teasing would have reduced his confidence which he needs to build and other students' opinion also would have raised several questions in his mind about sexuality which needs to be explored. Take Care and continue the way you are. Best, Sneha Bhat

Assertiveness training can help your friend. You can consult a psychiatrist hey evaluated about his d condition and start the treatment.