I have two green iguanas - hopefully I can be of some help. I'm no expert on
Iguana iguana by any means though.
First I'd like to say you shouldn't dose your animal by yourself. I suggest take a fecal sample to a vet for a float, to see if there are any parasites in his stool. Your vet can then give appropriate medication, Panacur is one of them. I would definitely recommend doing this if you know he is wild caught.
The best foods for
Iguana iguana include squashes such as Butternut, Acorn, and Kabocha, alfalfa, green beans, dandelion, collard, and turnip greens, papaya, banana, mango...I feed my iguanas these foods along with other fruits and veggies for a little variety. I do feed my iguana lettuce, it has lots of water in it if nothing else, which isn't a bad thing IF you have lots of other nutritious foods in it's diet. This is my favourite iguana-related website:
Food Information Chart You've likely seen this if you've been on Green Iguana Society (good site) but I'll reference it because it is awesome, haha.
However, you adults look quite healthy from what I can see...and it sounds like you have your feeding down...is your iguana lethargic? You mentioned UV lighting but not heat lights, do you have one? What are the temps in your enclosure? Does he bask under his UV? How MUCH do you feed him daily? Did you have your other iguana that young?
Both of my iguanas are rescues, and the younger one had Metabolic Bone Disease. He is quite small as well - to say "size doesn't matter" applies more to adult igs than a juvi than should really have a healthy growth rate. To say it simply, he is sick, so he doesn't grow properly.
PS...Love the outdoor enclosure!