Absolutely not appropriate for a 12 year old. Songs of Fire and Ice books were written for adults. Each book contains graphic sex scenes (and many, lurid descriptions of woundings, death, maimings and torture). A movie rating would be a very hard "R" to an "NC-17" - each book is bloody and sexy. And it's hard for me to imagine that any 12-year-old (no matter how advanced) could meaningfully follow or appreciate the plots and hundreds of characters without being in over his head. These books are as complex as Hunger Games and its sequels are simple. Readers, please note: I'm neither criticising nor judging negatively Songs of Fire and Ice. They are excellent reads that border on true literature. I'm just saying I can't image the 12-year-old for whom they are appropriate.
Yep. Hunger Games vs. SOIAF is like apples vs. oranges. I was a very mature reader at age 12, and the first really graphic book I read at that time was IT by Stephen King. Fell in love with his books from then on. I honestly probably would have been bored out of my mind reading these books back then, but it wouldn't have been too much adult overload for me either. Watch a few episodes of the HBO show. If you think it's something you'd let him watch, then sure let him read it, if not, don't let him read it lol.
ASOIAF is probably too graphic for most adults; first because most don't read any more unless it's a blurb on the interweb. I won't send my library of R. Jordan' WOT series to my nephew because I feel it to be a bit over the top for a 10 year old. Why not give an 8 year old a subscription to Playboy or let him surf porn sites at leisure? The language and graphic writing - excellent from an adult POV - used in ASOIAF depict a reality that most sci-fi fantasy glosses over, especially given the fact most is in medieval settings when human rights were null. Kings and queens were the ultimate in power and GRRM makes it realistic. If you know anything of history you know the dark ages were exactly that. Easter bunny versions don't do justice. When "off with their heads" is written graphically correct it makes for good reading but excludes those who aren't psychologically capable.