How can I use "have to" in question tags? Which one is correct? Can I say it both ways?
He has to come alone, doesn’t he?
He has to come alone, hasn’t he?
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In OP's example (where he has to means he must) it's not valid to use has he? as a "question tag".
That's because has in the preceding statement is a special usage that's effectively a completely different word to normal usages. It's even pronounced differently: not only is it always stressed - most native speakers normally enunciate it as hass (with an 's', not a 'z'). And hass he? is a completely unidiomatic question tag.
In other contexts, such as He has ten fingers, hasn't he?, it's fine to repeat the "statement verb" in the question tag, but even there I suspect most native speakers today would probably use does he?.
You use the former. The latter stands for an answer involving the present perfect form.
He has to come alone, doesn't he? (doesn't he have to come alone?)
This sticks with the negative form of the have to which in this case is the third person.
He has to come alone, hasn't he?
This question tag works when invoking the present perfect form:
He has come alone, hasn't he?