Thank you for your helpful response!
Just wanted to post an update. The vet gave us 2 doses of panacur 1 week apart. It's been about 12 days since the second dose and since then I saw A LOT of dead worms coming out with his poop. Pretty nasty. Anyway, today I saw the worms moving. I think Iguana needs more medicine because the problem is still there. I don't know how long the panacur stays and works in the body but I assumed it's about 1 week since you space doses 1 week apart. There's no blood in his stool like in the beginning which is good. He eats well too.
I'm planning on taking a sample to the vet tomorrow or the next day and hopefully he gives us more panacur.
I found this article about parasites in reptiles and they suggest to give smaller doses of panacur for 5 days instead of spacing a full dose a week apart: http://www.reptilesmagazine.com/Reptile-Health/Disease-Management/Reptiles-Nematode-Parasites-Treatment-Fenbendazole
Should I mention it to the vet or he will get mad because I tell him how to treat the Iguana?