First, you should use because instead of as. As confers the sense of happening at the same time, and confuses the sense of the sentence, especially since you have other time issues that are extremely confused.

Had been collecting is correct if he stopped collecting.

Has been collecting is correct if he is STILL collecting.

…travelling always fascinated him. is correct.

travelling always has been fascinating him is incorrect–it needs a preposition.
travelling always has been fascinating to him <– correct

Even correctly written, wouldn’t one travel because travel is fascinating? But that does not logically imply that he collects postcards because he loves travelling. That’s a whole separate hobby. I hope this isn’t a school assignment because it is a singularly bad sentence.