Can I reason with Dr. Mobius?

The New Vegas Wikia notes a potential bug with this quest (verified on the PS3 release, although I imagine the same script issue occurs across platforms):

If certain choices are made in dialogue, it is impossible to reason with Mobius and killing him becomes the only option, even if the correct skill checks are completed.

They suggest attempting the Repair check as a way to un-bug the dialog tree and get the "non-lethal" options back. If that fails, you'd probably have to reload an earlier save.

Past this quest, there's no real interaction with Mobius, so unless you really want a particular outcome from this quest, I'd probably suggest moving on if fixing it would be painful. There aren't any achievements, perks, items, etc, you'll miss out on.