。
I can't trust what he says.
Literal
I [topic-は] his [possessive-の] words [object-を] trust-can-not.
信用する ('to trust, have faith in, credit') + potential → 信用できる + negative → 信用できない. Distinct from 信じる ('believe'): 信用する is more about reliability/credibility ('credit'), while 信じる is about holding a belief to be true. 彼の言葉 ('his words') — specifically what he says, not his character.