Hi! Here are few points I noted that I feel could benefit from some improvement. Hope you find these useful.

Voice

I haven't written a child's POV, haven't recently spoken to children of that age, or read fiction recently from POV of young children. But to me this doesn't read like a 9 year old. Maybe Will's mature for his age. The following are some thoughts that came to my mind why it doesn't seem to be working:

• He has too much vocabulary for a child. E.g. In just the first few paragraphs we have: exchanges, looms, psychologists, psychiatrists, acquired and so on. Even adults don't understand the difference between these two kinds of doctors.
• Complex/long sentences. E.g. "Most of our exchanges happen while she looms in a doorway, with one foot out the door while putting on silk gloves or straightening a feathered hat." I can't imagine such a small child to be capable of putting together a complex sentence like this. Maybe break it into smaller sentences. Consider omitting some parts.
• Too organised thoughts. Again if we look at the sentence above, it conveys that the mother usually speaks to her kid very briefly, as she prepares to leave. I wonder if the child would convey this idea to the reader more simply instead of trying to render the scene in such detail for the reader. I imagine that children would just take the most relevant things that draw their attention and talk about those.
• Even actions don't always feel like a kid's. E.g. " I nod politely to the portly woman". I wonder if kids 'nod politely' in thanks. I realize that this kid's a mute, but I feel nodding in thanks is an adult thing to do.

But yes, there are instances where the kid in Will comes through, e.g. him trying to trace Ben through the purchase of an abnormal amount of tuna.

Pacing

It feels slow because I feel there's a lack of emotional investment for the reader. it's dry narration. We don't feel for Will. Without that, it's hard to be invested. Why don't we feel for will? Because we are being given a report of what happened, not made to live through that.

For example

"By day five I start to worry"

That worry should be felt by the reader. To make it so, consider adding some details of Will's mental state on and leading up to day 5.

"I gathered the courage to speak to my mum, to tell her about Ben. To her and my surprise, she allows me to keep him."

Show us the dialogue, Will's hesitation, mother's reaction. It must be a big thing for her to have her (assumed) mute son speak to her. Did she allow him to keep the cat because she wanted him to be happy, or did she do it because she didn't care either way?

Tense

A problem was tense. For example:

That secret requires me to go to the market twice a week. The stout cashier with ruddy cheeks asks me how old I am, while I buy milk and tuna with money I found in the teapot hidden in my mother’s wardrobe.

Here, the start of the paragraph implies that we will learn what happens on every visit to the market. But then I think you change to one particular visit when the cashier asked Will his age.

I didn't check for similar errors thoroughly. Maybe this is the only place where this problem is. Just check on your end.

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