Doesn't this sentence mean that we talk with him about a thing.
If it is about one thing it should be "èn ting", isn't it?
I've always thought English speakers say "talking to him" rather than "with him".
We say both.
Would a reversed word order like vi snakker om en ting med ham be incorrect?
Not really, but that's a very unusual way of phrasing it. 'med han' is a more important part of the verb than 'om en ting'.