At no point should we outright state "I'm a Necromancer, Omo." Doing so will only scare him.
Instead, we should tell him our story. How one day we went fishing and the fish came back to life. How we went to sleep one night and woke up from a nightmare to find that our bait had risen from the dead. How we discovered that the only way to keep from accidentally making undead was to exhaust our mana every day. Tell him that we're scared and we don't want to become a monster like the necromancers are, but we're also scared of telling anyone because they'd want to kill us. Explain that we've managed to learn enough to keep any accidents under control until we could destroy them, and that we hope that we can do something good with the power, something that will convince the world that we're not evil, not dangerous, and that, until then, we plan to hide our powers and keep out of trouble. If he hasn't attacked us yet, answer his questions as honestly as possible without incriminating ourselves. We've not done anything really wrong, so that should be easy.
Further, the golem thing probably isn't the best idea. Omo is sure to be weirded out by the idea that we've been using necromancy behind his back the whole trip. If he wants proof, and only if he wants proof, offer to raise a dead bug or something small like that. The golem can come later, when we're telling him about our plans to use necromancy to fight necromancy and defeat the evil necromancers by undoing their magic.