He didn't say he wanted a hard-on all day, he wanted a semi-hard penis. I think the idea
is to have a big long thing hanging down his pants leg so it shows, so other people can
see it. I have done this a lot and its fun to watch other peoples eyes linger on your long
bulge. But one does need pants that are kinda stretchy so they fit kind of tight around
your penis. I think you just keep your mind on wanting your penis to stay big, if it gets real hard, then ok, it will start going soft after a while, then you keep thinking
about it and it starts getting big again. But one thing: you are going to get a big wet
spot down there, but if you don't mind, then it is ok.