He began using it in 1967 as a member of the Yardbirds--and beyond. Here's two versions, three years apart of the same song, two different bands.
1967: "I'm Confused" (Dazed & Confused)" with the Yardbirds, before Zeppelin. Keith Relf, vocal, harmonica; Chris Dreja, bass; Jim McCarty, drums. You see and hear the bow at 2:27.
1970: "Dazed And Confused" with Led Zeppelin, Royal Albert Hall, London
Obviously he had mastered the techniques. But he did not conceive or pioneer the idea, much as he'd like the credit.
UK rock guitarist Eddie Phillips began bowing an electric guitar in 1963. He continued to do it as a member of the London band Mark Four in 1965 and when that band evolved into the well-regarded but underrated group The Creation. Produced by Kinks producer Shel Talmy, they had much in common musically with the early Kinks and the Who. But despite successes in Germany, their British success was modest and their US fame nonexistent. Given his extensive studio work, there's no way Page would not have known about Phillips's recorded work.
This is a live TV performance of their first single.
1966: "Making Time"
1967: "Painter Man" The Creation's biggest single (# 36 in the UK, Top Ten in Germany)
The Creation's best work was recently compiled on the 2 CD set Action Painting, recently released by Chicago-based Numero Records, covering their 60's recorded output in detail. The 46 tracks include including material by the Mark Four. Phillips continues to play (and bow) his electric guitar with a new incarnation of the original band. Page, too, retains the bow in his bag of tricks. So does Jonsi of Sigur Ros, Lee Ranaldo of Sonic Youth and Radiohead's Jonny Greenwood. This is Greenwood in action.
November 12, 2012: "Pyramid Song" -Radiohead, Sydney Australia
For the record, regardless of what Page says, Phillips opened that particular door.