Why does he do this ?

My 4.5 yr old becomes fixated on other ppl names. He’s been doing this since he learned to speak, any thing I ask him his answer is someone’s name. Idk it’s kinda of hard to explain, I’m gonna explain the best way I can. He can answer questions perfectly fine but his first initial response is my brothers name . Me and my family laugh about it but I’m actually concerned. If I say what do you want to do after school, he’ll say uncle simon(my brother) and I have to tell him to quit playing for him to actually answer the question correctly. Or if I say what did you love about school today he’ll say uncle Simon and then he’ll laugh. I’ll hear him in his room by himself saying uncle Simon this uncle Simon that. I wasn’t as concerned before as I am now. I’m more concerned now because he has found another name that he is doing this with. He’ll call me sophia( which is a girl in his class) or if I say it’s time for bed he’ll say it’s time Sophia. I don’t think if it’s the person that he’s infatuated with because he has little to no relationship with my brother and from what I see and hear about his friends at school Sophia is not one of the ones he’s close to. It’s almost as if he can’t control himself. I’ll ask him did he finish all his food and he’ll say uncle Simon and like grab his mouth like he didn’t mean to say it. I hope it makes sense but idk why he does this .