In this circumstance, can I say "There you go!" or "There you are!" instead of "There it is!"? Are they always interchangeable?
No, these are not always interchangeable. In this case, it is better to say "there it is" because he is specifically referring to the appearance of the smile on his face. If you said "there you go" or "there you are" in this context, it would be less specific and would sound a little out of context without the "it". However, in a context where the object or action is explicitly described, you can omit the "it" and just say "there you go".