Absolutely not. Those free to play games really pile on all the tricks to get you to part with your money and kids are even more susceptible to it, so you have to step in and put limits in place IME. £48 for a skin is ridiculous especially for a mobile game. It's not as though you're getting a whole new game, where, perhaps, it might be worth spending £48 (even that is a lot of money I think). To charge that much for something that adds no content at all is akin to a scam.
FWIW - it will be on a model where the virtual coins/currency you get in the game can either be "earned" by lots of boring grinding away at the game or you can put in real money to advance quicker. They won't really expect anyone to buy that shark but most players who add money will use the money they put in to advance in the game in other ways. The different priced items in the shop are just there to tempt you. It's unfair on younger kids who have this expectation that life should be fair and goals should be reachable, but the top items in the shop aren't meant to be reachable, they're just like a golden holographic carrot dangling on a stick to get you to play more (and watch more ads)/buy the currency to spend on the lesser items.
He's probably wanting to buy it just because it costs the same amount as what he has, and he doesn't really understand the value of the money - DS1 is very much like this as well. He was all set to go and spend €12 on a bloody skin for Fortnite to send to his friend the other day, because his friend made a jokey/hopeful comment that everyone else should buy him skins. He doesn't really have anything else to spend his money on so it doesn't feel like a loss/waste to spend it on something like that. As far as they are concerned, they want it, they have the money = job done. Anyway, it prompted a discussion about virtual items and we agreed a maximum cap for the spending on virtual stuff. A downloadable version of a game that would have 10 years ago been a disc is separate from that, and so is paid DLC like extra levels for example, but content for a game that doesn't really add anything, like skins and dances and shark upgrades are subject to the cap and can only be bought from that "pot".
I also agree with comparing and contrasting with other things they can get. DS1 wouldn't be motivated by a toy catalogue because he's not really interested in toys and hasn't been since he was about 5 or 6, but he is interested in sweets/junk food, he's starting to get into expensive trainers/clothing brands too (he's 11 so a bit older than yours) so I'm dangling a pair of Adidas trainers or a hoodie in his eyeline as well. And of course what he really wants is a super duper computer or console - although this feels really out of reach, we have pointed out to him that if he really has nothing to spend money on, save it up and eventually he'll be able to afford his own big TV or console or computer parts or whatever. Trying to do a small kind of interest-on-savings experiment with him to help with this too.
But in short, no, don't feel bad - if it helps, imagine that your son is interested in Pokemon cards, and the 12 year old boy who lives next door has offered to sell him a "super rare, super powerful" card for £48. It's not a card which sells to collectors, it's a common one which is worth about 50p resale. You know it's a scam, the 12yo next door knows it's a scam, your DS naively thinks it's a great deal. This is the same, it's just you can't point to an ebay listing for this shark showing that it's worthless IRL.