Hi there, and welcome to the forum!
It's been kinda slow around here lately, but I still check in to see if anything's new. It's good to hear the ig is FINALLY in good responsible hands. Yes the diet was horrible... I don't have a green ig, but I do have a 2+ year old spiny tail ig and a baby rhino ig. Their diet is basically the same (especially for the rhino, the spiny gets occasional mealworms). We feed 2 of either mustard, collard, or turnip greens (sometimes watercress) and add in "spring mix" greens as well as 2 types of squash daily and sweet potato often, which was recommended to fatten up the baby rhino. They also get fruit a few times a week, mainly berries, but sometimes mango or papaya. You didn't specify the new diet, but I must say our igs are both fat little lizards! You may also want to try misting him several times a day to rehydrate him incase he is dehydrated (this will also make him look thin). I would recommend daily baths, but not untill he has acclimated to his new home, as the handling may cause too much stress on him right now. Just make sure his humidity and temps are correct for now, and the misting and new diet should help. Spraying the food with water is also a good way to ensure hydration.
As far as the tail goes, it was probably dropped at some point in the igs sad past. New growth is often darker in color as you described. It is nothing to worry about if this is the case, but will probably never be the same color as the rest of him. It may also be shorter, and therefore reduce his total length a bit. A more reliable way to determine if he is the proper size would be to measure snout-vent length. Considering his past and the estimated size you describe, he sounds very small for his age, unless you were saying he was 13" snout-vent. Like I said, I don't own any greens, but our rhino is only 5 months old (we have the hatch date) and already 15" total length, and rhino's are typically not as long as green iguanas when full grown. Poor care will decrease full grown size, but I would not worry about that. The most important thing is that you now provide him with everything he needs to be happy, healthy, and grow to his potential given his past.
You didn't say the size of his enclosure, but it is important to note that bigger is better, and green igs need a lot of height. No, they do not grow to the size of their enclosure, but will grow faster and be happier/healthier given more space and the proper dimensions.
I highly recommend you schedule a vet visit once he has had some time to adapt to his new loving home. Blood work and a fecal exam will tell you if you have any parasites to worry about.
Good Luck, and let us know how it's going!
P.S.-Here's my little fat rhino today...she is such a little sweetie!
