How long has this new pattern been going on?
Does the towel and plastic method work to contain the messes?
Perhaps his body is expelling something that irritates his bowels. If so, this could be a short term problem.
Maybe he has a new gut flora/fauna. If it's a benign one, he will eventually adjust to it. Or his immune system might eradicate it. Similar to the way humans eventually get used to wild water (that's not infested with giardia). I don't remember whether iguanas can contract giardia.
One thing to try to counter sudden bad pooping behavior is by shopping for proven live cultures of probiotics at good health food stores. I picked the probiotic with the largest number of organisms, and fed 1/4 tsp twice per day. The thinking is, the bolus of additional beneficial organisms can overwhelm the unwanted organism(s). Lots of people on this site (15 years ago when this site was very busy) used that method.
You can get probiotics balanced for birds at an avian vet. It's thought that this is closer to natural beneficial iguana gut bugs than those balanced for humans.
I always used the human probiotics. They never to my knowledge caused more severe bowel problems.
My first response was always to take a fecal sample to my exotics vet to check for anything bad.