The boy - speculation (Spoilers warning)
I just finished watching Solaris (2002) and had a thought. I know that the credits list the boy as "Gibarian's Son" but its not like movie makers haven't mislead audiences before. I'm thinking that the boy might have been the child that Rheya aborted.
The only in-movie evidence that it was Gibarian's son is Snow's statement. BUT Snow turned out to be a Solaris manifestation, so its information is to my mind suspect.
Kelvin's Solaris manifestation is based on his loss of his wife. His wife's suicide was in response to their fight over the child/fetus. Given the irrational way the human mind works, I can see Kelvin thinking that "solving" the issue of children with his wife could have stopped the fight. So some part of his subconscious imagined a son.
There is also the issue of Gibarian is dead, so why would his Solaris manifestation still exist? (The problem with this line of speculation is that how would Snow's manifestation still be around to mislead Kelvin? Maybe Snow is Gordon's manifestation! )
At any rate, while its not an important issue for the movie I was just curious about other people's (and other manifestation's too ) thoughts on the idea.