1) Don't tell him it sucks, convince him it sucks.
2) Let him go ahead and try it. He'll learn more from trying and failing than anything. He'll also come up with several new startup ideas in the process of working on his first one.
I frequently struggle with #2 when "teaching" young children. How much do you let them flail when you know their idea is destined to fail? Even if you can see that an idea is clearly a flop, nobody believes you when you tell them. Gently nudging seems to have the best results. Let them pursue their idea but nudge here and there to lead them to find the flaws on their own. "I saw a guy try something like this once and this is one of the problems he ran into..."
2) Let him go ahead and try it. He'll learn more from trying and failing than anything. He'll also come up with several new startup ideas in the process of working on his first one.