I see you have been offered other decent information here but one thing I dont see listed is UVB. For optimal and improving health, this is a must and given the condition , I recommend a 150 watt Mercury vapor bulb. These also produce heat and it is best if the are mounted above the iguana , roughly 15 inches from the igs back, also they require a ceramic socket light fixture, both available at commercial pet stores.
I know you may have limited options of items at the shelter but there are just some things that must be provided. Quality UVB, a cage large enough to create a basking zone of roughly 95 to 100 and a cooler zone of 75 to 80. Iguanas belong to the group of animals that thermoregulate in order to help with proper digestion and absorption of nutrients. Without that become malnourished and food can sit in the stomach and rot.
The mass is more than likely and abscess, hopefully easily fixed. The patches and sores you mention could be a couple of things from burns, old stuck shed or fungal infection. Some of that can be difficult to treat depending on severity.
Please come back and keep us posted on his condition , also I may know someone with experience in Pa thats looking for an Iguana.
10 years is not old for an ig. I have one that arrived near death at age 17 that is now 22.
They are amazingly resilient when proper care is offered! Good luck!