story identification - Looking for surrealist/ridicule book about a boy with a wingsuit

I've been looking for the name of that book for a time now, I remember it to be quite funny. Here is what I remember about it:

• I read it in French.


• I remember that it was a book for kids (from 8 to 12 years old I think)


• It was read to me when I was little by my mother (so I read it between 2005-2010)


• The main protagonist was a boy who lived in some kind of sewer system or underground, he goes to the surface at the start of the book.


• I think he had a family member living with him, possibly his grandpa.


• There is a city on the surface, I think the boy went there to scavenge/steal supplies or other stuff.


• He used some kind of handmade wingsuit to hover from roof to roof. I don't remember if he could properly fly with it.


• I remember that cheese was seen as a very valuable ressource.


• There was a ridicule/surrealist feel to the story.


• There were water cows. Actual cows that lived in the water. Not manatees.



• There were walking cheese roulette which were hunted, I think there is a hunting scene at the start of the book. I think that the boy witnessing the scene put him into trouble. This would also explain why cheese was so valuable


• I think there was some kind of cult/scientist which created a large abominations using cheese.


• I remember that the main character spends some time with talking pirate rats.


• The talking pirate rats own a laundry business on a ship.


• The rat's captain is elected democratically each year or so.


• I think the protagonist and the rats overthrow the cult at some point but one of the rats falls into a pool of cheese which makes him grow into some kind of severely obese rat, like really big.


• To save the obese thing, they huddle him onto their boats and then, using some kind of tool, they pump the fat/flesh/cheese out of the rat and send it into ladies's butts. (They also launched some kind of operation to make big butts popular.)

Answer

This is the first volume of the Ratbridge chronicles by Alan Snow, entitled Here be monsters! (2005, published in France in 2008). French and original covers:

From Goodreads:

Welcome to Ratbridge. But beware—for there is skulduggery afoot. Young Arthur has fallen foul of the appalling outlaw, Snatcher, and is trapped alone in the town with every way home sealed.

Meanwhile Snatcher and his men are working tirelessly in secret on a fiendish and dastardly plan to take over—and destroy—the entire town. With the help of Willbury Nibble, QC; some friendly boxtrolls and cabbageheads; Marjorie the frustrated inventor; and the rats and pirates from the Ratbridge Nautical Laundry, can Arthur thwart Snatcher’s evil plans—and find his way home?

You read that in French, so this review might be of interest. Here are some key points and their rough translation:

Bon-Papa vit au milieu de ces créatures, dans un sous-terrain aménagé. Il a pris sous sa protection le jeune Arthur qui remonte régulièrement à la surface pour trouver de la nourriture.

Pops lives with these creatures, in a refitted underground. He's taken young Arthur under his protection, and the latter often comes up to the surface to scavenge for food.

Fits the "family member" you were describing.

Le jeune Arthur parvient à s'échapper mais la bande de Grapnard lui dérobe la machine qui lui permet de voler [...]

Young Arthur manages to escape, but Grapnard's crew steals his flying machine [...]

Fits the wingsuit.

Vous croiserez ainsi dans ce livre une vache marine d'eau douce, des fromages sur pattes, des rats pirates devenus gérants d'une laverie automatique [...]

In this book, you will encounter a freshwater cow, pawed cheese, pirates rats turned managers of a laundry shop [...]

If you're interested in how I found it, I searched for "roman vache eau rat fromage" ("novel cow water rat cheese") in Google, and this review came up in the results.