When it comes to adding cats in a home, there is a process that is used (here anyway) that makes the transition as smooth as possible. Unlike humans, and part of the problem with cat owners, is we tend to think of these critters as people with little fur coats and four feet- and they are NOT! Because of that, we try and rush them through the introduction process,believing they can reason things out. Unlike a home crowded with people, a cat can't come up to another cat and say, "Hey stop eating all the food in the refrigerator will ya?" Or "Don't hog all the hot water!" Instead they hiss, they growl, they scratch, and will even bite, unless they are left to figure out for themselves how to get along.
If your cat Lily got outside accidentally and took off- she would meet up with other cats soon enough. That is how they survive. Admittedly outside, there are more ways for a cat to be free of another cat- but inside surrounded by four walls, there is no escape. So they posture, they growl, they hiss and their stress level goes up which is why your little boy got hissed at.
The very best way for cats to get to know each other, is to put between them a door that is not solid. Cat proof screen doors are available. My husband built a really nifty portable door out of pvc pipe and chicken wire. This allows the cats the opportunity to see each other, smell each other, play with each other and no one gets hurt in the process.
Yesterday, I took the new five kittens out to the enclosure to let them explore. All six of my adult cats were out there, and the two latest rescued kittens were as well. I let the kittens out and stepped back to watch. I did not anticipate problems, and there were none at the time until Prowler who is very predatory stepped in and started stalking the babies. I put Prowler in the house, and let the kittens have about an hour of enclosure time. The adults were watching intently but there were no confrontations and I finally gathered up the litter and brought them back upstairs.
This morning, Wink and Gypsy are at each other throats. It is a delayed reaction from the kittens. They knew the kittens were here- as they were upstairs in the cat room, and there is a chicken wire door up there, so they have seen all the babies. But now the babies are in their turf, and they are a bit riled up. Gypsy hissed at both Mike and I this morning, but we understand why she is upset, and we let her get it out of her system. She is fine now, but her and Wink are uneasy with each other- but they are getting along.
Your cats should not be sharing anything right now. Not time with you, not litter pans, not water or food bowls, not toys. Provide them with ample ways of getting off the floor and out of each other's path. Cat ramps, cat condos, window perches, clear off a bookshelf. Determine who your alpha is and be sure and put their food bowl down first then put the other bowl down. Don't approach these cats and expect them to jump in your lap and purr out their content, until they learn to get along. And give them time- be on their time schedule, not yours. They will adjust- it just takes time-