For males, mating season brings increased alertness and aggression toward other male iguanas. Aggression is often generalized, causing acting out to the owner. They often pace within their cages. Sluggishness is not a normal response to breeding season.
If your iguana is actually a female, gravidity in it's late stages causes sluggishness. They become gravid even without a mate, like hens laying eggs without a rooster. But 1 1/2 years old is a bit too young for becoming gravid. Could (s)he be older?
Is his cage possibly too small? Does it lack a good hiding place? For an outdoor cage, it should be at least 2x2x2 meters for an 18 month old iguana. He needs 3x3x3 meters minimum when he is 2 to 2 1/2 years old. It's hard to give a big enough cage for indoor use. Part of it must get full sun for at least 3 hours per day, and have a large shady spot for him to retreat to when the sun is hot.
It's possible he has a disease. Parasites are the most common illness for iguanas, causing sickness and malnutrition and even death. He ingests them with the food you gather for him. Intestinal gram-negative bacteria cause illness similar in action to parasites. Panacur is good medicine for parasites. You can probably get it from farm supply stores, sold for use with cattle. You would have to figure out the tiny doses for your iguana. Baytril is a good antibiotic for reptiles and may be available from your local farm store. It can be given per mouth(per os, or PO).
Some of us might know the actual reptile dosage per kilo of body weight. Can you afford to get a gram scale that is accurate to within 0.001g? It might be risky for you to get one, if you know what I mean. We can do the math to find the dosages if you can obtain the cattle medicine.
Dehydration causes similar illness. You can use a 3 cc plastic syringe to supplement his daily water intake. 3 cc per day in divided dosed would be good for your 18 month-old ig, and is not harmful.
That's all I have for now, I hope he recovers on his own.