Realize there is something that I don't think you know about: Your iguana is very likely to be in Change-Related Stress. While this is happening, he may sulk, hide, lay motionless, and refuse to eat or drink. Learn about it in this PDF:
Lettuces have virtually no nutrition, for iguanas or for humans. The only exception is romaine lettuce. It does have some nutrients. Escarole, which often considered to be a lettuce, is good for iguanas.
The best way to wean your iguana off of romaine is to very slowly add little bits of good food. Maybe a tablespoon of minced collards or kale per two cup serving of romaine. Serve it on the side of the dish. Give him a week to get used to it. If he accepts it, then slowly increase the added food, then after two or three weeks, add an extra food that is nutritious, such as grated hard squash or escarole.
Then, very slowly increase the good foods while very slowly reducing the Romaine. IMO this process will take about three months or more. Or faster depending on how your iguana reacts to the changes.
To make sure he's eating enough, feed him more than he can eat, so there are always some leftovers. When he starts to finish his entire food serving, increase the amount until there are some leftovers again. The theory that works says "Iguanas will not eat so much that they overeat." Where they ancestrally live in the tropics, during most of the year there is bountiful good food for them, yet the wild ones do not get problems associated with overeating.
Get ready to use a LOT of patience (-;