Him is accepted as well and equally likely. Note that the English it can easily be any gender in Czech. Here we must allow masculine and neuter, like masculine návrh (proposal), or neuter rozhodnutí, usnesení (resolution). You can also vote against a city (město). All of that is proti němu and against it.
See my answer to taalf
“We will also be voting against it” sounds quite unnatural. In Czech hlasovat is indeed imperfective, but you cannot mechanically translate that using the progressive tense.
Hlasovat can mean both to give a vote (dát hlas - budu pro tebe hlasovat, dám ti hlas) or to be continuously voting (hlasovali o tom hodinu). In the future tense we say budu hlasovat. You can imagine that voting is a process that takes some time. But in English one normally uses the simple tense.
Note that the progressive tense is accepted as well, because the continuous voting meaning is possible. But less likely, in my opinion.