Certain newer versions of programs allow users to SMS from their computer, newer versions of both Yahoo and AIM allot them to SMS you.
Verizon has a program that compiles Yahoo, AIM and MSN into one program. If you'd be interested in that, you can PM me and I'll try and send you the URL.
Otherwise you're stuck to only if they have new versions of IM.
if you have unlimited data, then why not just use a data based IM program? whether he has data or not has no bearing on what program you use. if he has his IM's set up to come through as text (like through AIM's IM forwarding), then whether you are using a data program or not won't matter on his end.
however, since you are on AT&T, you should be able to download their mobile IM client (also known as OZ messenger) which communicates through text.
Have him set up his AIM screen name to auto-forward to mobile when not online (or have him set up a separate AIM screen name that is always set to auto-forward). What this will do is allow him to be shown online with the mobile icon and any IM's sent to that screen name will come through as a regular text message that he can reply to. absolutely no data plan is needed as all communication is done through SMS.
You, on the other hand, since you have an unlimited data plan, can use any of the AIM options available to you that you would then use to IM that screen name that he has set up.
