Here is my take on the situation. First off, I'd get the cat re-checked for a urinary problem or possible infection from the "meatball" removal. Second thing I'd do is change the food you're using. Most, if not all, grocery store pet foods contain TONS of ash, which is very bad for any cats with possible urinary problems. Even those labeled "for urinary tract health" can be very bad. You'll have to start buying the more expensive foods, but if it works, you won't mind. I've always fed my cats Science Diet, and they've never had a problem, but those grocery store foods can wreck havoc on sensitive cats. Also, I think another problem is that you received the kitten too young. 5 weeks is WAY TOO young for a kitten to be weaned from it's mother, and can cause MAJOR bahaviorial problems. The kittens really never learn how to act (the example you give of the kitten with your son in bed is a perfect example) properly since they were too little to be removed. Unforunately, if this is the case, there is usually little you can do, with the exception of medicating the cat. Kitty prozac might help in this case, but it can get expensive and might not help in the long run. You just might have a scenario that you had a kitten that was too little to be "alone" and when another larger, more experienced cat was placed in the picture he got scared and defensive, since he didn't know what to do, since those skills were not yet learned. I will not take a kitten into my home until they are at least 12 weeks old for that exact reason. There are only babies as well, and do need to learn some behaviors from other kitties, and they haven't even had a chance at 5 weeks.
First thing I'd do is take the cat back to the vet for another exam with regards to the "meatball" removal and infection. While there, talk to the vet about the obvious behaviorial problems this cat has. Maybe you will be able to find a solution, but if not, you might have to find another home for this cat, since it truly might be better off in a one pet household.