There's nothing inherently "positive" or "negative" about either of these options. Neither one implies agreement or disagreement. You can use one or the other with careful inflection of tone or in the right context to suggest positivity/agreement or negativity/disagreement, but that has little to do with grammar.
The "is he?" tag is a rhetorical request for confirmation of a fact that was just stated. This is a form of parroting: repeating a statement, possibly in paraphrase, to indicate that the statement was heard and understood. Consider the following exchange:
A: "I'm afraid he's not feeling well."
B: "He's sick, is he?"
The second speaker is paraphrasing the first and repeating the statement. The only information this adds to the conversation is the fact that the second speaker heard and understood the first. It's possible that the second speaker is using this form mockingly (as if to say, "Oh really? I doubt that."), but this is impossible to know without context or analysis of the vocal inflections used.
The "isn't he?" tag is a genuine request for confirmation of an idea that hasn't been clearly expressed in the conversation yet. Consider the following exchange:
A: "I'm afraid he's not available."
B: "He's sick, isn't he?"
The second speaker imagines something (that the reason the subject of the conversation isn't available is because he's sick) and is asking the first for confirmation of the idea. Again, any deeper implications cannot be known without more context or vocal analysis.
It's not unreasonable for the "is he?" form to be used as a genuine request for clarification rather than a rhetorical one. Gramatically speaking, this is perfectly fine. In practice, however, I think it's avoided as the "is he?" tag is commonly used to imply mockery or disbelief. But again, this is not gramatically relevant.
In short, the correct answer depends on what meaning your teacher ultimately wants you to convey. Does the teacher want you to express understanding, or request clarification? In the former case, only "is he?" is the correct answer. In the latter case, either option could technically work.
Based on your description of your teacher's instructions, it seems that they were looking for the former case, where only "is he?" is the correct answer.