They seem perfectly happy eating meat. Honestly, the most questionable thing we've done so far was draining vitality from a still living tree.
But yes, once we are beyond the edge of the forests, we will truly resume our experiments in earnest. There will be one last thing that needs to be done, though, first. Perhaps a days travel past the boundaries, we will tell Omo there is something we need to show and explain to him. Remind him that he knows us fairly well, and we wouldn't have kept something secret from him this long unless it was important - and to hear us out, in full, before making any decisions. But we hope he decides to stay with us - we need him, not just for his abilities to fight and survive off the land, but to be our ethical compass and a reminder of what we have to lose should we err. Tell him you will explain the rest once camp is set.
Once we've set up camp in a well secluded area, ask him to hunt us up something for dinner while you prepare your story. Once he returns, start at the beginning. Explain how your powers revealed themselves - and how if you didn't use them, they manifested themselves on their own. Ignoring them wasn't an option, so you attempted to learn about them. You don't need to go into details about the specific experiments, simply tell him of the things you learned, and of coming to terms with this. Of your internal conflict, and renewed determination to do something good with what you have realized is a gift. Pevo has told us that not all Necromancers are evil - merely, humans expressing the magic on their own without guidance corrupts an already dangerous discipline even further. Our nature, our life experience, allows us a different path - though we are wary of the dangers, and we wish to rely on Omo to keep us on the straight and narrow.
Then explain how, with these powers, we believe we have a chance to do what the Life Mages could not - remove the taint from our elven homeland. Perhaps it will be beyond our abilities, but if nothing else there is surely something valuable to be learned. If we can learn how to turn our powers in such a way so as to counter the undead, even better - we can use those powers defending out people, should it come to that.
You can demonstrate the truth of your powers by animating the corpse he brought back from his hunt, controller it, and then killing it. Explain that the corpse is still good to eat - you tried it yourself, previously - and that nothing here is wasted. You simply have the ability to use the death for protection and discovery as well as sustenance. If elves have prayers of thanks or whatever, do those properly.
And then... we'll see what he says.
And we do, eventually, intend on revealing ourselves to them - when the time is right, but not before. When they can be sure of our intentions, and we can stand on our own two feet, such that if they DO reject us, we can carry on without growing bitter or resentful.
Ask him if he has any questions, and his opinions on things. Reaffirm that you are interested in his desires, and his recommendation, and what he thinks.