I am concerned. My 75 year old uncle will purposely not eat a meal, saying he's full, but an hour later will eat lots of sweets. Can something b wrong?

See below. Maybe his memory is not so good? Not unusual behavior in some elders. He might do better if you remove sweets and provide more healthy snacks. However, at his age life is more about quality than quantity and if there is no specific conraindication to candy and he understands risks and benefits would you stop him by force? You can try reasoning with him, but be prepared to lose.