I got kittens when they were very young, and initially made sure they had lots of warm places to sleep - blankets in lots of corners - to allow them to feel safe. NO MILK! Cats cannot tolerate lactose. Whiskers have a specific kitten milk which is safe for them but normal cows milk can lead to sickness, diorreah, and damage the stomach. At 3 months they should only need water.
Feed the kitten dry food, specifically for young kittens. Mine eat Red Mills or Royal Canin (think it;s blend 36). The dry food helps ensure no plaque build up and keeps teeth clean and healthy.
Gradually move the kitten box away from the area to where you want it.
Worm the cat every 2 weeks while it's a kitten, then it can change to 3 monthly. Drontal or one of the more specialised ones are good. Fleaing should be done monthly.
Highly recommend neutering/spaying - as well as calming them down, it also reduces risk of cancers etc. considerably.
I kept my cats locked out of the bedrooms whilst young, so that they weren't used to going in there. If the cat is going in your sons room, make sure you teach your son to wash his hands regularly.
Lots of gentle handling to get them used to you.
If you have other animals do gradual introductions. I was lucky, my dog loved them and they are best friends now
(cats even come for walks with us over the fields!)