Make leaflets and distribute them within 1/2 mile of where he was found. That way, if any locals have heard from the owner about this, they will contact the owner.
Iguanas never get very far from where they escaped. Normally only several hundred yards at most. Most only get a few doors down the street one way or the other, and also they cross back fences, bringing themselves to another street.
If you don't hear back from a dense leafleting, this iguana was most likely dumped, or lost by a traveler.
Is there an Iguana Rescue in your town or a nearby town?
Iguana care is very complex and counter-intuitive. Any new guardian must read up on iguana care. This is a very good, peer-reviewed set of articles which most successful iguana owners have used:
Scroll down to see all the different articles.