Sometimes lizards need to be re-accumulated even ones that have been in captivity for a long time. You mentioned force feeding him, sometimes that can do more harm then good. Lindsay pointed out to me that if it is a real dominate male that force feeding can break his spirit as the act of force feeding him is dominating him.
Get a digital gram scale and weigh him every 2-4 weeks. As long as he is not losing large amounts of weight I would not force feed him.
You mentioned that he is pooping. If he is pooping he is eating something when you are not around.
Stress can cause a Uro to go off food. If you have him in a glass aquarium cover all 4 sides. If you have him in a tank with glass just on the front cover that. Do not handle him for a couple months until he starts eating again.
Check your basking tempters with a Temp Gun much more accurate than a disc thermometer or a digital probe thermometer.
http://tempgun.com/120-140 degree basking and a gradient of 75-85 degrees on the cool side.
If your Repti Glo 10.0 Desert is over 6 months old you may look into replacing it.
Try adding edible flowers to your feeding mix.
Dandelions, Hibiscus, Hollyhock, Rose petals, Nasturtiums, Dahlia, Day Lilies, Geranium, Pansies, Violets, Cat's Ear, Birdsfoot trifoil, borage flowers, arugula flowers, and mustard flowers, chrysanthemums, marigold, Etc.
Try dry foods like:
A mix of, uncooked green peas, lentils, various 15 been soup mix (remove large red kidney beans) grind up in a coffee grinder. You can add bee pollen to the mix. Lots of Uromastyx go nuts for bee pollen. You can find bee pollen granules at most health food stores.
You might also try Mazuri Tortoise Diet.
I have had lots of luck feeding white hulled mullet. Lots of people use it as substrate but I keep a small bowel of millet in there cages and my Uro's seem to love it.
It could be the winter slow down. Depending on what species you have they might even stop eating for a couple months while they brumate.
Keep us posted as to his progress.