OK, I'll bite. If it's open source, then why not just tell us what it's about? Is it a front-end for the next generation of casino slot machines to sell to Indian casinos?
In such a rapidly developing community someone might take the idea and finish it before I have the chance to.
Surely that's a good thing? That's pretty much the spirit and purpose of open source software.
Sounds more like you want to call it open source but dual license for personal vs commercial use in order to make some money. Of course, this is fine, many projects do this, but in this case, there would be no interest in using for personal use - who's going to set up a gambling site that they run on their local machine to gamble against themselves? As soon as you allow others to gamble, it becomes commercial.
The other problem if you try to dual license is that the open source libraries that you use in building this project may no allow relicensing.
Just because he's planning to open source it after release doesn't mean he doesn't have a valid reason of not wanting to release the idea now. Open source later doesn't mean he can't make money off of it. And just because it's open source doesn't mean the only way he can make money is by licensing the software. Apparently you don't have much of a grasp of how software development works.