Hi.
Sooo many times our favorites turn out to be cockerels!

So cool that you found him a home.
I wonder if you might think about waiting until one of you Silkies goes broody and adopt her a chick, preferably two. I adopt sexed chicks from the feed store of whatever breed they have that I like. They often get shipments every two weeks of new breeds. Hopefully your Silkies will be a little less broody than mine! But that would be quite an amazing experience for you son.. I think.
But my original suggestion was going to be consider waiting until next year when your current girls will molt and stop laying eggs for a while.. so the new pullet(s) would still be laying through winter, usually.
My Easter eggers are mix between calm/great and flighty but still good. They are among the most active of the MANY breeds I have had. Like them a lot. Not my version of quiet, but not usually an issue. And REGARDLESS of breed will vary by individual. Couple of my Silkie gals really announce it when they are getting ready to go go lay. They are pretty cute. One of my EE preached hour long sermons at the top of her lungs on the highest pulpit she could find before everyone ignored her and she slipped away to lay her egg without giving her nest away. One of the funniest things ever! Interesting and clever. I've raised well over 100 birds of different breeds and any can be that way. It just depends. One of my Lavender Orpington made this poltergeist sound that the first time my hubby heard thought one of my goats was dying (distant sound) or being attacked by something. Not a peep from her almost twin sister. I think you get the idea, just get what you like and see how it goes, in my experience. Also, Orpington are pretty & calm and hang out close enough, but not my version of lap "friendly". I feel meh about Brahma, think they are heavy eaters and large birds in general. I'm hoping Speckled Sussex will rock since that's what I chose for breeding next year and working to build stock currently! Two ladies that have made pet status and past many, many cuts to the flock are a barred rock and an Easter egger both hatchery stock. Good layers, great personalities, lap friendly with people they know, and good flock members... in addition to being very hardy. But this is MY version, many others will have a different experience.
Another thing I need to mention is that if adding a new bird that has been with another flock... quarantine is suggested for 30 days, far from your other birds. In case something develops that the other person might not have seen yet. That being disease, parasites, or illness.
I will say, in my experience... 3 is actually a really good starting flock... but 5 (as in 3 now and 2 more next year) is even better.

Here I go trying to teach you chicken math!

Don't follow me.

It goes like this... 4 chicks minus 1 cockerel= 14. 14 chickens minus 2 birds to a friend eager to get started... equals ... 12 chickens, 4 chicks, 2 goats, 1 rooster, "some" eggs in the incubator... and a cow on the brain... maybe some quail or ducks..
We don't talk numbers anymore!

Best wishes with your flock.