It's a question I've been asked by adults a few times, so I assume it's on the minds of many more. It's a question I genuinely appreciate, since it means the person asking it wants to interact with C. So, here's my answer:
Yes, please talk to C. Please ask him questions. Please engage him in play. Yes, please treat him as you would any other child.
Be prepared: he may walk away from you while you're mid-sentence; he may not seem to notice you or be interested in you; he might even seem distant or aloof.
But know this: he's not rude. He's not trying to be hurtful. In fact, he actually does want to be able to engage with others. I know, I've seen how he lights up when it happens.
Yes, there is a little boy inside C. Please, by all means, feel free to talk with him.