I admit it's been a while since I saw this film, but is there any evidence in it to show that the kid is actually Supermans? all I remember is a piano flying across the room, but that it wasnt shown how that had happened, it was more an assumption that the kid had done it, and it could have just as easily been the boat they were on lurching that did it?
Well, the piano scene, we see the piano fly across the room at a really fast rate. The shot cuts to the kid who has his hands outstretched where the piano was. The piano could have spontaneously moved, or the boat could have rocked, but considering there was no jarring movement to force the piano to move a such a great speed, this is unlikely.
Also, his Asthma suddenly stopped, which links to the flashback earlier in the movie that Clark gained perfect vision when he learned how to fly.
Also, there is the fact that Lois tells Superman and Superman comes to the house later to give him the speech his father gave him. So, unless Superman was under a horribly mistaken assumption, the kid was his.