Like other animals, an iguana requires socialization. Plus, they seem to have a short memory. So if you don't handle him at least once a day he may revert.
Has he had any recent exposure to dogs/cats/birds? These are his natural enemies and could traumatize him enough to make him turn surly.
Bathtime always seems to cheer the little buggers up. Let him have a good soak in the tub filled with lukewarm water every other day. This also does wonders for his color and helps him shed. He will leave you a little gift for your effort, though.