You're right that it's technically wrong to end in a preposition but in colloquial speech, almost all native English speakers would say 'Where did he run to' or 'Where is he running to?' or perhaps they'd leave out the 'to' altogether. (In fact I don't think I'd EVER ask 'To where is he running?' despite its grammatical correctness!) But I think that any of these versions should be accepted by Duo.