You should read Kenneth Pitts book The Pitt Report, to get an understanding of how he was received in the 60's.
He had many people championing him; in the press and among peers and friends. Pitt also said he did have a UK fan club from the 60's and I recall in the book, he even had some letters from fans overseas asking about Bowie. Rest assured there were many people who had a clue that David would become huge; but alas not so much in the record companies. I read one interview once where an industry insider who knew Bowie in the 60's said that he and others just couldn't understand why Bowie wasn't famous yet (in the 60's). They could all see his talent, but it was a mystery why he didn't break earlier. I guess it just wasn't his time and it's a testament to Bowie's determination, that he carried on as long as he did before finding stardom with Ziggy Stardust. I think if it weren't for the many people who championed him during the 60's he may have given up long before.