You can do some observational things to try to avoid a vet visit.
In my experience this water behavior you describe is not common but is normal, even if it did only recently start. It's natural for the occasional individual to submerge and stay on the bottom for a while, especially if they are spooked or have suddenly decided they don't like the tub. Submerging is the ultimate hiding behavior for igs who can't run away. They feel absolutely invisible. I've seen it in several of my rescues. Sometimes an ig who is spooked by water, such as in tub training, to bloat itself up with air so it can float, sort of a survival mechanism. One of mine did that and got an emergency vet visit out of fear his intestines were gassing up to a possible harmful degree, but it turned out to be an expensive laugh.
I really doubt he aspirated anything.
Something may have spooked him recently, like a change in his surroundings, or seeing something scary outside the window, or an especially loud bout of thunder. Or if a cat or dog has been sitting near his cage, watching him. Or he has found a mirror or a shiny surface which reflects his image making him think there is a trespasser in his house.
We always want to see our iguanas eat well, but it is true that their eating varies for natural reasons. This is breeding season, which causes stresses, so both males and females may stop or lower their eating for a limited (seemingly very long) time. A low eating ig can easily go way over a month with nor harm, and healthy non-eating igs can easily fast for more than a month with no harm. It's much harder on the owner that the iguana.
I would wait it out for 3 or 4 more weeks, or until something bad starts up. Check the thickness of the base of his tail, and watch for it getting thinner, which is a sign of weight loss. If it gets markedly thinner, then he is becoming emaciated and needs a vet.
My policy was to always take an ig to the vet at the first faint sign something was wrong. It's actually cheaper to do that, to avoid the high cost of curing something that has dug in.
Are there any treats he is wild about? If so, feed those a few times a day for a while.