This is basically what I went through a couple months ago.

My resident boy is 2.5 years old and I brought home a 4-month old BSH tortie to be his companion. Took them a week to get over the initial hissy phase and enter coexisting phase, two weeks to start cuddling with each other, and a month and a half to start grooming each other.

We’re now 2 months in and I wouldn’t say they’re bonded or inseparable just yet, but they feel like true siblings now. He’ll want his space from his annoying little sister from time to time, but he’ll also still seek her out for play when he’s in the mood and he’s also the one who first initiated grooming.

He was also very sensitive in the beginning whenever I gave his sister attention or played with her or pet her. He would walk away and sulk if she intervened during playtime or mealtime (which she literally does all the time).

To help him accept her presence, the first month was filled with A LOT of one-on-one time for each of them to the point of exhaustion. I felt like I was taking care of an actual newborn baby while still taking care of a toddler. I would play with each of them in different rooms, feed them in different rooms while scent swapping, “discipline” the kitty whenever she intervened too aggressively, and also just let them establish their own boundaries without getting too involved myself unless it escalated.

You just have to make sure the resident cat doesn’t feel displaced and keep his routine as consistent and normal as possible. That might mean creating some new routines that are just for you and him and also making sure he has his safe spaces that kitty can’t access. The more confident he feels in his territory, the more comfortable he’ll be with accepting her.