It sounds like his mum, or perhaps the care team, or probably both, are afraid of hypos. Which is not unreasonable especially with such a young child! But I wonder if his mum has gotten so worried about injecting him with this rather powerful (in its way) medication and is overcompensating to make sure he's okay. It must feel very intimidating when it's all new.
When I was starting out, they sent me home with a meal plan - 30g carb per meal. I don't think I could stomach 90g even now, as an adult. Is that what he used to eat? Because that's what make me wonder if it might not be out of fear. Sounds like he might be a picky eater as well. I suspect Liam is right and they're starting him out slow while everyone gets used to it, will probably work to dial it in soon. When I was diagnosed, the protocol was to keep the new diabetic in hospital, learning how to inject, carb count, correct, etc. under supervision for two whole weeks before they let you out on your own! Of course that was 18 years ago and they'd never do that now, but the learning curve is still just as steep, so a lot of it happens at home with phone contact over the first few weeks. But definitely keep an eye out because it is a lot to cope with and they may have trouble taking it all in, especially without letting fear and anxiety take over.
My parents completely missed mine for months. Put the thirst down to me playing outside during the summer and the hunger down to a growth spurt. Our grocery store was an hour away (isolated community!) and it wasn't unusual for one or the other of us to need to stop at the halfway point, so no one really noticed when I started needing a stop every time, both directions. Wasn't until I went off to a birthday party that my friend's mother realized something wasn't right with me. So I think it's not uncommon for parents to miss it! Sort of a boiling frog thing.