The VAs are the one singing the character songs so yes, it's him.Thank you!!!
Edit: Question, I'm listening to the Hisoka OST right now, and its called : "Kyousou Requiem" and the person singing the song sounds EXAAAACTLY like Hisoka! Does anyone know if thats the same voice actor singing this song? I mean seriously, it sounds identical o_o.... pretty freaking cool!