The best way to have a great idea is to have tons of ideas, but without sharing these tons of ideas, you don't know which ones are the good ones, and which aren't.
Rather than just accept that his ideas were 'shot down', he should try to understand why a certain idea was 'shot down'. Maybe it was outlandish, wouldn't work because of things he hadn't considered, etc. etc. If he uses each of these experience to further hone his idea generation, he can look at each proposal of one of his ideas as an opportunity to get further down the track to the right idea.
I think a big part of that is in HOW he proposes the ideas. Rather than jumping in whole heartedly with "this is how xyz should be done", he could come at it with "I was thinking about the way we do xyz, and maybe it would be better if we did it by abc..." then he gets the feedback and gets to hone the idea.
I fondly remember why my favorite mentor became my favorite. I had never had a mentor before, but we were working on a large project and when we'd discuss ideas, he would say little things that would open up my thinking further, which I would grab hold of and push further again. I called it 'leapfrogging', where people were coming together to discuss an idea and each others ideas would progress the whole together further. There was no ego, as it was a truly collaborative environment where ideas flowed freely. Not every idea worked, but the experience was fantastic.