Some of his teachings are indeed present in the Upanishads. Especially, his statements about the Self, happiness etc. all do exist in Indian Scriptures. He was a maverick philosopher that shunned the traditional system of teaching.
Sure but that doesn't mean they were "based on ", i was mostly curious about that part.
afaik, they were just based on a pure common sense not based on Hinduism/Buddhism etc. There is a video on youtube of him talking to some Buddhist scholars where he goes into this.
Unfortunately this common misconception of miscategorizing him as "hindu guru" which instantly turns off most westerners, stopping him from gaining much wider acceptance.