Yeah, as a hairstylist... and a mom to two boys, I have "groomed" many men's unibrows. I don't do waxing, but the person in our salon who did, waxed preteens eyebrows, backs and such all the time.
Jut say non-chalantly, "Hey boys, I got this cool little trimmer thing that works on eyebrows and mustaches and stuff. Here's how you use it, you want to try it out? It's very popular among guys these days. When you get older, you can use a for real razor and dad will show you how to do that, but for now this works great!" Then, don't pester him if he doesn't want to groom it, just let him know the option is there if he does.
And it's true, men are more prone to grooming than ever. My friend took her 16 year old son... who is very manly, not feminine in any way, to get a pedicure the other day! He actually really liked it.
The groomer I am suggesting is easy, quick and pain free. You can get at CVS, Walgreens, Walmart... places like that for about $10:
http://www.hair-removal-world.com/men-groomer.htm
Don't use a regular razor and go straight down, it will remove too much hair in the middle and make his eyes look too far apart. A common problem with people who use razors.
Really though, if you haven't already, you are going to have to talk to him about deodorant and such pretty soon anyways.