If you're going to speak to your GI, you might also ask about broth and jello (gelatin). My son was also allowed these. As he took in all his formula overnight, he wasn't at all hungry in the morning but would take broth to 'eat' at lunch with friends (and school kept freezies for him). His dinner was broth again... it's not much but it's a savoury flavour so 'similar' to real food.
At 12, your son might be old enough to manage the insertion of the tube himself?? There's a youtube video of a young girl demonstrating an NG tube insertion... she does a great job in the demo, only thing I found is she is so precise, it seems like a long process. Once my son did it a few times (less than a week), he was inserting the tube in seconds (and removal is even quicker). Would give your son the flexibility to swim, etc. during the day without worrying about the tube.
Make sure you request a thin tube - my son's size was 6Fr. This is supposedly 'infant' size but my son was 5'10", 150 lbs when using it and there were no issues.
I'm going to tag Jmrogers and Clash - I'm pretty sure both their sons learned to insert the tube as well.
However, just know that some people have a tougher time - there are quite a few kids here as well who just weren't able to insert the tube on their own.
If you look under the Diet subforum (here in the parents forum), there's a thread on EEN - there's lots of info there as well.