You can repeat yourself that he has standing all you want, but standing is established by dame or injury. What is the damage done to Trump?
If Cruz should win in Iowa, Trump will have suffered “injury-in-fact” (direct not indirect injury) which is a requirement of standing, if, repeat IF Cruz is ineligible.
McCain, Palin, Romney and/or Ryan could have been granted standing to sue Obama as the only other people to receive Electoral votes and have a legitimate chance to be elected if Obama was to be found to be ineligible. That again would be injury-in-fact. But none of them filed suit or were the lead plaintiffs in a class action suit.