Another approach that has worked for me is to listen to his idea, and then "walk it through" with him, and mention in a subtle manner that "it would work better this way because. . . ." or "the problem I may end up with is. . ."
I find if I do that, and then finish with "but I don't know if that's something you could do/design/figure out", they rise to the challenge. And on occasion, his ideas have been of value too! Good Luck!