Tong is not a bixesual boy. Tong's character is a perfect example of a queer kid and compulsory heterosexuality, his dynamic with Donut is one side (by her). Donut's persistent attempts to capture his attention only underscore how emotionally and physically disconnected Tong is from her. He goes through the motions because he thinks it's what he's supposed to do, not because of genuine attraction. In his social espace he's believed to act like a "man" and for that he's peers (included Donut) dictates that a teenage boy must date the most popular girl to be validated. And all that vanish (gets more complex) the moment Mew re-enters his life, the fog of comphet is shown for us... His connection with Mew exposes how effortless, natural, and authentic mutual affection, touch, and physical attraction feel when he is finally living in alignment with his trues feelings. Comphet forces queer individuals to second-guess their feelings for years.
When with Mew he's so easy going, living and just Tong. He shares with Mew his weight and life problems his family lost and all his "complaints" with Donut, every interaction looks like a chore, like putting a mask just so he doesn't add even more on his and his parents plate. After all the "normal" at his age is to be with a girl.
PS. : Tong and Donut don't have a relationship. So whatever a cheating thing some people believe happens... It doesn't 😀