It's very possible that I'm completely over thinking things, so if that's the case here, I apologize in advance. Is there a way to distinguish between someone saying "this is my grandson's portrait/portrait of my grandson" in the sense of the aforementioned portrait literally being a portrayal of the grandson, and "this is my grandson's portrait" as in "this is a portrait of someone else that my grandson owns"? For some reason I feel anxious about this hahah
Technically there's no difference and both meanings are valid. In fact it can also mean "the portrait by grandson painted". Though, I believe, "portrait of my grandson" would be the default assumption for most people. Other meanings would probably require certain context or some kind of explanation.