The three methods for taking in water are PO(by mouth), SubQ(under skin), and IC(between the abdominal organs). SubQ and IC are far better than PO, and IC is the best. Some vets are OK with teaching iguana owners the Sub-Q method. It's easy. One other route is into the bone, called IO, but I can't imagine how that works.
Has the base of his tail become noticeably skinny? If so, force feeding might be a lifesaver.
About force-feeding. We have to realize that non-eating iguanas are not eating for a reason, like being really sick and nauseous, or severe pain. So forcing food has to be done as a last resort, and very carefully. You have to be as sure as possible that forced food is placed way back on the tongue, which triggers the swallow. More than 1 ml at a time might cause him to breathe a bit or two of food into his lungs.
I used to very finely mince collards and cooked Butternut squash, putting them in a 3 ml syringe with the end cut off, like loading a cannon. Sand the edges of the syringe's bore so it won't cut mouth tissues. Pack the minced food into the barrel. Open the mouth wide, put the end of the syringe far back, and push one ml or so onto the tongue. Give him a full minute to swallow. Make the first several feedings as gently as possible, as he will learn what is happening and hopefully cooperate.
I normally never suggest pulling down on the dewlap to pull the mouth open. Using a tongue depressor to press between the lips will make him open, but in his poor shape, teeth might break and cause infected gums.
It might help you to know that over the years I've pulled several near-death iguanas back from the brink by pushing food and water, but it takes time. One took an entire six months before she suddenly dove into a pile of food.