If you were to be honest, as I had to be when I was first corrected, the use of the term "frat" is more than just an abbreviation. I would suggest that you, like me, even though I have Fraternity ties from college, use the term derisively and not without a little big of contempt. There are "animal houses" out there, without a doubt. But they are the minority. As for the rest of the Greek system, both Sororities and Fraternities tend to seek much higher goals academically, socially, and in many cases, spiritually. The term "frat" is objectionable to them and as such, whether or not you agree, you should show the common decency to respect their request.
I happen to be very proud of his decision and his loyalty to his Fraternity (as long as it can co-exist with his biological family). Teamwork is highly underrated in our society today.