He learned to fly from Piccolo off screen after his 6 months survival training and before the Saiyans showed up.

I also assumed so, since at some point Piccolo mentions how his namek flight technique is faster and more efficient than Sky Dance (Crane Hermit's flight technique) or whatever Kami taught Goku, just assuming Kami didn't spill all the beans so Goku's initial flight skill is a mix of copied Sky Dance with the blanks filled by Kami's teachings. Hence why Goku still uses Nimbus while Piccolo keeps up with Nimbus and notes how it isn't a huge stamina drain for him, but for Goku it was still slower and more tiresome.

Oh, stamina drain. The whole reason why Piccolo didn't regrow his arm right away and the crux of both his and his father's defeat, they were evenly matched but blew up all their ki/stamina on flashy and destructive moves. I miss when endurance was a thing, they really had to be wise about their ki expenditure, and Roshi would berate anyone using ki techniques indiscriminately.

One old RPG game even enforces this, Goku and Piccolo's KI/MP gauge for a good while at the start is enough for only 2 kamehamehas, 3 basic ki beams or a single Makkankopoppotamus, so make them count.