Hi!
I just wanted to say that if your son was 2 months premature he is really only 6 months old. Before Marissa came into my life I used to raise money for the March of Dimes because I had a lot of premature babies in my family. For a little while those couple of months makes a difference, like he may be two months "older" than the "average" age that other children crawl, but be completely normal.
That being said, 12-13 lbs does seem small even for a 6 month old. Are you breast feeding or formula feeding? Is he not gaining because he is not eating or vomiting?
I can't really suggest anything to feed a 6 month old baby. The G-Tube would help with gaining weight, if it is a matter of not being able to get him to eat. But I don't know if it would do any good if he is a happy eater but losing it in another way (throwing up, loose stools).
Good luck! Please let us know how things go.
Terry