To be specific, he wore an outfit and squatted in the style of a slav squat for Halloween in 2017, which was a popular meme at the time. Yes, there was a pepe slav squat meme. But if you look at the hat he's wearing, it will match the hats worn by slav squat and not the pepe meme, which I think was actually a hoodie

EDIT: i guess I was thinking of a Halloween costume from another year