The designer of George's T-shirt reveals why these pictures are perhaps the sweetest yet

Credit: Matt Porteous/PA

Birthday boy Prince George just proved that he can pull off casual chic just as well as vintage hand-me-downs.

While the adorably cheeky royal is often dressed in traditional style clothing or even vintage items for official pictures and appearances, the occasion of his third birthday (which happens to fall just after a heat wave in the U.K.!) set the tone for his most low-key look yet.

Perhaps the only nod to the formal nature of past portraits is shown in his hair, with his thick blonde waves combed smartly into a side-part, no doubt by mom Princess Kate!

“He just looks like a normal, sweet 3-year-old in a casual T-shirt and a pair of Burmudas,” says Emily Cohen, founder of Sunuva the beachwear brand worn by George in one of two looks captured by photographer Matt Porteous and released by Prince William and Kate on Friday.

The British brand – who specialize in high-end stylish SPF swimwear – tells PEOPLE they couldn’t be more thrilled: “To dress him in Sunuva for the official pictures is beyond flattering and incredibly exciting for us.”

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The $36.70 navy stripe 100 percent cotton T-shirt sold out almost instantly on the website, and the company’s office was inundated with calls. “We have had a big uplift in traffic on the site,” Bunty Stokes, managing director at Sunuva, says, adding, “Luckily, we have managed to get hold of some extra stock from a supplier, but the phones have been going crazy!”

In another picture, in which George is casually walking through the gardens at his rural Norfolk home, he is wearing an outfit from Pepa & Co, an online retailer that specializes in Spanish clothing (little sister Princess Charlotte is a fan).

Prince George Is Turning 3 Years Old!

“He looks absolutely adorable in our clothing and we could not wish for a better ambassador!” Pepa Gonzalez, founder of the brand, tells PEOPLE, adding that the couple clearly favors Spanish brands for a reason. “In Spain and at Pepa & Co, we believe in trying to keep the essence of timeless clothing for children and enjoy seeing our children look like children.”

The Breton whale-print sweater, which retails for $38.55 has also completely sold out and there is limited stock of the taupe shorts, which retail for $47.20.

Although barefoot in most of the relaxed and care-free style pictures, George can be seen accessorizing with his current go-to plimsoles, the Hampton Canvas Nantucket shoes from Trotters – which at $19.70, have also almost sold out.

The pictures show adorable George standing on a rope swing and kneeling on a picnic rug, smiling as he feeds his dog Lupo ice cream.

“They are very sweet and it’s just a big more low-key and casual – [Kate]’s mucking around with him on the swing in the garden, having a great time, like any other mom,” adds designer, Cohen.