An appropriate question tag for "He's too weak to walk."

A couple of days ago, I sat for an English exam. There was a question there that asked for the appropriate question tag for the sentence "He's too weak to walk." I answered "isn't he?" but my teacher claimed that the correct answer should be "is he?" since the sentence expresses a negative meaning. Frankly his explanation made no sense to me.

Is he right? Am I missing something? What should be the appropriate question tag for the sentence He's too weak to walk.?

When we're asked to add question tags in tests, we are expected to add tags that agree with the assertion, for example, It's a great day, isn't it? Question tags that genuinely express doubt whether the sentence is true or not aren't expected.

"He's too weak to walk"

Is he ? suggests that the person asking is doubting, or at least needing confirmation of the assertion.

Isn't he? suggests the person agrees with the assertion.

The point is, they have different meanings and should be used depending entirely upon whether or not the person asking agrees or not with the assertion.

Wikipedia has an overarching approach:

Balanced vs unbalanced tags

English question tags exist in both positive and negative forms. When there is no special emphasis, the rule of thumb often applies that a positive sentence has a negative tag and vice versa. This form may express confidence, or seek confirmation of the asker's opinion or belief.

She is French, isn't she?

She's not French, is she?

These are referred to as balanced tag questions.

Unbalanced tag questions feature a positive statement with a positive tag, or a negative statement with a negative tag; it has been estimated that in normal conversation, as many as 40%-50%[2] of tags are unbalanced. Unbalanced tag questions may be used for ironic or confrontational effects:

Do listen, will you?

Oh, I'm lazy, am I?

Jack: I refuse to spend Sunday at your mother's house! Jill: Oh you do, do you? We'll see about that!

Patterns of negation can show regional variations. In North East Scotland, for example, positive to positive is used when no special effect is desired:

This pizza's fine, is it? (standard English: This pizza's delicious, isn't it?) Note the following variations in the negation when the auxiliary is the I form of the copula:

England (and America, Australia, etc.): Clever, aren't I? Scotland/Northern Ireland: Clever, amn't I? nonstandard dialects: Clever, ain't I?

As usual, you can't say a usage is ungrammatical until you've decided on your particular grammar.

And even then, the intersection of 'grammatical according to say CGEL' and 'what my teacher wants' will doubtless be shaded grey.