wi connect from imac to pc

I have an iMac which is running mountain lion and have set it up to share the internet which is connect to the imac by ethernet. My son has an older pc and is unable to pick up the wi fi signal from my iMac. He has an iphone and it connects right away. The signal strength from the iMac wi fi is 100. appreciate any suggestions.

Does the older PC even have a wifi card in it?
Historically, it would be pretty rare that any off-the-shelf PC had a wifi card in it from about 3 years ago and older.

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.

Also, the Mac will have to be on -- not sleeping -- at all times during the sharing. As you can see, it's meant to be temporary.