Kiam li falis, la knabo ŝiris sian ĉemizon.
In this sentence, sian seems to refer to li because I think this situation the boy is grabbing the man's shirt to prevent him from falling down.
But it seems that sian may refer to la knabo also.
Is it true that sian may refer to either li or la knabo according to the context?
Kiam li falis, la knabo ŝiris sian ĉemizon.
The boy ripped/tore his own shirt, grammatically. It could very well be that li refers to la knabo. To disambiguate that:
Kiam la viro falis, la knabo ŝiris sian ĉemizon. (de la knabo)
Kiam la viro falis, la knabo ŝiris lian ĉemizon. (de la viro)
The original phrase looks like the man while falling tore the boy's shirt, where one may formulate it as the boy tearing his own shirt (non-actively, in that process).
In a subclause si always refers to the subject of that clause (see PMEG), here to the boy.
If you want to refer to the main clause, you can use li like Joop Eggen suggests or alternatively tiu to refer to another actor than the subject in the subclause.
Kiam la viro falis, la knabo ŝiris ties ĉemizon. (de la viro)
