I think he would have seen much contemporary Buddhist observance as entirely disconnected from his teachings. That doesn't necessarily mean he would have scorned it. There is one sutta in which a follower of a rival teacher was convinced by his teachings and wanted to follow him, and the Buddha advised him to continue respecting his former master. There are also important suttas in which he stressed that his own teachings are not "true" in an absolute sense, and religious teachings should be valued according to the extent to which they are helpful. I don't know the historical status of those suttas, but historically it doesn't seem common for first-gen carriers of the faith to retroactively exaggerate the uncertainty and doctrinal pragmatism of the founder of their religion. Many modern Buddhist teachers (notably the Dalai Lama) express great respect for the ability of other religions, including Islam and Christianity, to inspire and nurture spiritual progress in their followers. So he might look at modern Buddhists reciting nembutsu or leaving offerings at Buddha statues and say, I can't see any connection between these practices and what I tried to teach, but they seem to be helping people a little bit.
His attitude might also depend on how seriously he took rebirth, which was widely believed in his time and place and takes a central place in some of his teachings. It's a point of great controversy among contemporary Buddhists: was rebirth a part of the teachings because it was coincidentally an accepted view in the time and place where Gautama taught? Is it an inseparable part of Buddhism? Both, neither? Can you reject rebirth and still preserve the essence of Buddhism? If Gautama showed up in the present day and said, "Oh, practically nobody believes in rebirth anymore? No matter, I can teach everything I know to people who don't believe it," that would rock the world of Buddhism. But vice-versa probably wouldn't make much of a difference, and neither is going to happen anyway so we'll have to live with the controversy regardless.
" It's a point of great controversy among contemporary Buddhists:"
among contemporary western Buddhists, maybe, who have a hard time wrapping their heads around the concept, (which is understandable, since Christianity - at least the mainstream versions - which was the dominant European religion for centuries has no corresponding concept).
In Buddhism as practised in Asia (Thailand, Burma,Srilanka, India, Nepal, China, Japan etc) it is not controversial at all, and is quite mainstream, with next to no "controversy" around it.
His attitude might also depend on how seriously he took rebirth, which was widely believed in his time and place and takes a central place in some of his teachings. It's a point of great controversy among contemporary Buddhists: was rebirth a part of the teachings because it was coincidentally an accepted view in the time and place where Gautama taught? Is it an inseparable part of Buddhism? Both, neither? Can you reject rebirth and still preserve the essence of Buddhism? If Gautama showed up in the present day and said, "Oh, practically nobody believes in rebirth anymore? No matter, I can teach everything I know to people who don't believe it," that would rock the world of Buddhism. But vice-versa probably wouldn't make much of a difference, and neither is going to happen anyway so we'll have to live with the controversy regardless.