If you have a good chance of getting him to a reptile vet in the morning keeping up first aid might be the thing to do.
He could be suffering from a vitamin B deficiency or metabolic bone disease or an infection and the vet may be able to help him.
As iguanas may reach 30-50 years of age or more commonly in captivity 15-20 years its probably not old age issues.
Depending on the diet and habitat he is kept in though it might be a kidney issue or one of the above listed problems.
You can read up on kidney issues here
http://www.anapsid.org/iguana/kidneyfailure.html
and about other common issues here
http://www.animalhospitals-usa.com/reptiles/iguana_health.html
Hope this helps you!
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