[arandr0x]

It looks to me like your hire messed up something about this guy's plans and he set out to minimize damage as fast as possible. Since he had contacts and you didn't, he won (they usually do). Look, when you start at a new place, especially as a manager but even for ICs, it's because someone high up thought they needed an extra person (or a different person). And it's quite likely not everybody agreed with them. It's not really personal (although sometimes these people are OK with the hire but had a dream of a different skillset and don't think yours matches), but it's dangerous.

In those situations you have two options -- find someone he is close to who's more positive about you that you can leverage into brokering a truce, or try to use him for information. (You can straight up ask braggarts what they would do in your place, which usually makes it obvious why they aren't in your place already, in addition to giving you future bones to throw him to keep the hostility in check)