The fight would suggest he didn't learn it until the end, given there were many opportunities he could have and should have used it prior such as whenever Piccolo took to the sky during their fight. It's also pretty telling that he was carrying the Nyoibo with him upon his reintroduction, something he wouldn't need to do if he could just fly down to Karin-to. His surprise at Kuririn being able to use bukujutsu is also a bit much if he were fully capable himself, considering he wasn't aware how long Kuririn trained on his own away from someone with the skill like Tenshinhan.
Goku most likely had the basics of it from observing Tenshinhan and Popo, with it only being through fighting Piccolo and sheer desperation that he was able to completely master it.
Bukujutsu doesn't seem like the sort of thing you can just pick up in the spur of the moment.
Bukujutsu is seemingly easier to grasp than forming one's Ki when you're shown how to do either, at least using Videl as an example, and Goku completely mastered the Kamehameha by the time of the 21st TB despite not likely having any opportunity to use it after Pilaf's Castle. Goku is simply incredibly skilled at learning new techniques, and it's not as though bukujutsu is that complex when considering it's the most widely used Ki technique by alien races (including a pre-potential unlock Dende and the lowest level Freeza Soldiers that rely more on arm cannons than Ki blasts).
