Lack of interest in food is quite common in geckos, and may be the result of enviornmental stressors or changes. Some Geckos naturally reduce their food intake in cold months or fast for periods of a week or so.
Tail shortening may be related to Autotomy. The connective tissue of the tail is structured to allow the tail to break off at determined fracture points, spaced along the length of the tail. This happens as a defense to predators, in response to stress or can be accidental.
If your Geckos share a tank, your one year old may be responding to the change in his enviornment, competition for food, hiding spots or feel crowded. Tail loss is often the result of bullying between tank mates. Often the signs of bullying are not obvious to the owner, as the bullying can take place at night when the owner is not about. Signs of bullying including scuff marks or a mouth-shaped bite mark, on his head, along the body and tail.
If you house your Geckos together at this time you should seperate them, especially if they are both males.
Whether it is due to bullying, jealousy or some other change causing stress the reaction is enough to explain his symptoms. Under stress geckos tend to hide more, have darkening/dulling of the skin, lose their appetite (or refuse to eat, cease to enjoy attention, become lethargic, and can appear skittish and alter normal patterns of behavior.
Because he has lost a significant amount of weight he should be checked for parasites. These are natural to the Gecko, but grow out of control if his immune system is compromised by stress.
If the source of stress is not relieved he may continue to lose weight and muscle tone, dehydrate, and develop systemic infection (bacterial, viral, fungal or parasitic).
Hope this helps you both; good luck!
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