Further to bobtomay's post - I would actually recommend against doing WiFi sharing from the iMac for several reasons.
WiFi sharing is really intended to be a stopgap measure for impromptu connections. It's not particularly secure, and range is drastically limited.
For $30 at any WalMart, you can pick up a wireless router that is vastly superior in terms of security, range and speed. You also won't be wasting system resources on routing, which is what you're asking your iMac to do.
If your son's PC doesn't have a WiFi adapter, you can pick up a USB wireless adapter for about $20 that will make this much more simple to setup. In short, what you're attempting to do is needlessly complicated for very little net benefit, and in fact, could create a significant security problem.