。
What do you really think of him?
Literal
Really him [possessive-の] thing [object-を] how think?
彼のことをどう思う ('what do you think of him?') is a natural way to ask for an opinion about a person. のこと softens the directness — asking about 'the matter of him' rather than directly about 'him.' 本当に ('really') pushes past any polite hedging.