Hingy, I forgot to add
ADIUM to your options.
If he has a PC and you run a Mac, this software will allow you to ichat. It's free.
I've had it for a while now, I don't use it much since I acquired a VoIP phone and X-Lite softphone, but it does work well, and it's free.
If you go the Skype route, a lot depends on the amount of calls and conferences. A pro account with a Skpe-in phone number will cost you about $65 the first year, $95 the following years. A stand alone VoIP # or a Skype # is about $60 per year.
The number is used as a local call. You would purchase the number for your brothers area code and he could call you directly. If he already has a Skype account he could buy the additional number in your area code, and you could call him with a 7 digit number. I had a number in Estonia 2 years ago for friend that works for Cisco, I could reroute the numbers to forward to calls to my cell or to his brothers house, anywhere really.
The cheapest avenue for you to take if video chatting is all you need is to download Adium, and connect to his selected video conference software.
Good Luck
<edit> to add this link for creating an iChat AV link with a PC.
http://www.mvldesign.com/video_conference_tutorial.htmlOptional software>
Trillion $25.00