You can write 'is singing' but in this context 'sings' is better. If you want to be specific about his singing here, rather than his singing generally, then 'sings it' makes it specific.

He sings is the correct one. For the third person/noun (he/she/it) we must add 's' after the verb. I hope it helps you :)

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1. To be honest, the tone is actually quite good, but the way he sings kills the song. ✔ ------- You're reporting an event that happened just now, (or in fiction, for example, the same as No.2), and you're sure what you're saying is true. 。。。。。。。。。。。。。。。。。。。。。。。。。。he sang it killed the song. ✔ 2. You're reporting an event that happened a time ago. 3. 。。。。。。。。。。。。。。。。。。。。。。。。。。he sing kills the song. ✘ Not grammtically right.

Sings is correct, or you can also write "is singing" in this sentence.