I think it would be okay to ask if he knows how YOU can track back but, unless the offensive e-mailer actually does something that brings him into your uncle's attention officially, I would really be not ask your uncle to do something unethical (possibly even illegal).
well.. it is very possible but you might want to check the legal issues about it first ;) anyway you easyly get back-tracing softwares to tacrk your senders ip etc.
eMailTrackerPro 8.0d is one you might want to look into
hope i helped
Copy the full headers of the e-mail and forward them, along with the original message, to abuse@ whatever ISPs the message either originated from or passed through. You may have to do some digging (no pun intended :) to find out what the domain name is, since some MTAs only use IP addresses. nslookup is a good start.
Don't waste time or money buying any software for this, as you don't need to.
Secondly, there are no "legal issues" about reporting abuse. If someone is sending you abusive or threatening email messages, the first place to start with is their ISP's abuse department. Local, State and Federal authorities won't step in to do anything, either because they just don't know how, or something like this isn't important enough to spend their time on.
Trust me, you'll get farther by going to their ISP first. I doubt your uncle will be able to assist too much with this, unless he's a computer security expert with lots of contacts. Just being a "Fed" isn't enough..