DY: He wants to know why an Eved can’t volunteer like an Eino Metzuveh V’oseh like a woman can. The simplest Chilluk is that an Eved K’na’ani is still a Goy and a Goy can’t volunteer certain Mitzvos because he would be impinging on our Bris with HKBH.
That would be incorrect, though. The simplest answer is that an Eved can volunteer to learn Torah (according to the Rambam), just like he can volunteer to do any Mitzvah according to the Rambam.