What would make a man who's almost 500 pounds with normal thyroid function and no other health problems not lose weight when actively trying and changing eating habits for over a year? low testosterone?

Weight is related to sleep duration & quality. During sleep 2 hormones Ghrelin (gain weight) & Leptin (lose weight) are produced but with short or fragmented sleep, there is an imbalance. Talk to your primary care physician about a Home or In-Lab Sleep Test. If diagnosed with Obstructive Sleep Apnea, a dentist can help with an Oral Appliance. https://www.sleepfoundation.org/sleep-apnea/weight-loss

No that wont be the reason. What you need to do is to see an MD who specializes in weight problems as 500 lbs is excessive and you will need help to lose it. Good luck