。
He says whatever's on his mind.
Literal
He [topic-は] thought thing [object-を] mouth [target-に] does.
口にする ('to say, mention, speak of,' literally 'to put in the mouth') is an idiom that can mean either 'to say, mention' or 'to eat' depending on context. Here it's 'to say.' 思ったこと ('what (he) thought,' relative clause with こと nominalizer) is the object of speech. The sentence describes someone blunt or unfiltered.