First, what is the best way to socialize him and how do I slowly gain his trust?
Get him used to your presence first.
Just hang-out in the room where the enclosure is, and do NOTHING to him. Read a book, watch TV (not loud), or play a game, yada yada yada. Do this for a week or so. The only interaction between you and him is bring food, water, and clean cage. He will associate you as "the food-provider". This is a positive association.
Once you feel he is comfortable with your presence (less head-bobbing, hissing, etc), interact with him inside the enclosure. That is, gentle petting, talking (get him used to your voice), but NOT grabbing of moving him. Just get him used to your hand. If he bites, clean and disinfect your wound, but repeat with more care. If you stop, he wins, and will associate biting with successfully getting rid of you. Get him used to your touch, associating you as a neutral, non-threatening presence. Another positive assosciation.
With time increase level of interaction, making sure it is always a positive experience, and maintain your position of control. With positive reinforcement, you'll get positive results. Keep in mind, as cute and cuddly iguana owners portray their iguanas, they are still wild, exotic animals not truly meant for 'domestication'. So don't give up, and be patient, VERY patient.
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help him to drink more?
Some people put icecubes on top of the mesh of outdoor enclosures so that water drips onto the leaves or drinking bowls, hinting a natural way for the iguana to get water. Also, iguanas get water from the fresh veggies and (some) fruits they eat. Soaking does help, and putting small water dishes around helps too. If the animal is thirsty, it will drink.
Misting is an excellent method as well.
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I've been cleaning with regular bathroom cleaner and then spraying it with lysol then rinsing it out, is this ok?
If the bathroom cleaner is anti-bacterial (if not, get one that is), the combination with the Lysol should do. Some people use bleach, but if the cleaner is anti-bacterial, and you give it a good deep scrub, and you let the Lysol sit for about a minute to truly disnifect and sterilize, you're fine. The Lysol or Clorox wipes are convenient too.
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His previous owner said the only thing he would eat is commercial bought iguana food, is this healthy for him or should i try to get him to eat some "iguana salad"?
This can be a problem, since iguanas are creatures of habit, and they detest change. Commercial iguana food is not bad, just not guaranteed to do the job. Of course, natural is always better than processed. You can mix a bit of fresh-minced veggies (appropriate for iguana nutrition) in the food, and slowly and gradually increase until the animal is accustomed to eat more fresh veggies than "iguana-chow". Keep it varied and interesting (I know, sounds ridiculous and crazy, but you'd be surprised). If all fails, stick with the commercial food, make sure it gets the proper vitamin supplements, and heck, if it's hea;thy and not skinny, let it be.
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Since his cage is outside on cold days I let him free roam in my bedroom is this ok?
Yes IF your room is "iguana proof", does not have things that can endanger your pet, or things your pet can knock off causing damage. No places to escape to (vents, etc), rip, get stuck, electrecuted, fall, die, etc.
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I have a 15 month old niece that lives in the house(who doesn't touch the iguana but has been in the same room with it), how exactly is salmonella transmitted and what precautions should i take to make sure this doesn't happen?
A person gets salmonella if coming into contact with the animal's feces, or possibly other bodily fluids, tho feces are the prime cause. Iguanas are not exactly "clean". Their long tails can easily get soiled as they are dragged over feces, claws get dirty, vents, and so forth. Children/babies put things in their mouths, hands included. Hands touch about everything. Iguanas walk over about anything. You make the connection. A clean animal in a clean cage in a clean environment will not pose a threat. Before letting the iguana free-roam, wipe its body from vent to tail (and if feeling brave, hands and feet too) with baby-wipes. Yup! This will already minimize the chances. If the baby goes in the room AFTER the iguana has been in it, prior to letting the baby in, spray Lysol or wipe Lysol (or Clorox, whatever you got) around to further decrease contamination. Iguanas and small children, particularly babies, don't mix, and so it is preferred the two are not in the same room. However, if both are kept monitored, chances are the iguana will sit in one spot and bask at a heat-source, and the baby will best mind its business with things other than a large lizard with serrated teeth, sharp claws, and a whipping tail.
I love the measurements of your outdoor enclosure! Excellent!
Outdoor enclosures are THE best thing an iguana can have.
-IJ