I don't think you'll get any useful advice here. You should be able to come up with your own idea if you want to be a successful businessman anyway. That's how it works usually. Depending on other people's ideas is not a good way to start things. You'll mostly get generic crap like "start a casino", "start an exchange", "sell stuff" etc.
I agree. The ideas he gets here may not be enough to start a successful business. But still, I think brainstorming with other folks could give him new perspectives he hadn't thought of solo. Someone might not hand him a million dollar concept outright, but could ask thoughtful questions or offer input steering him the right way.
A few possibilities in hand, he could mock up barebones landing pages using that free domain. Itd give him a taste if potential customers dig those concepts. Not failproof market research but better than theorizing in a vacuum.