get lab blood work done. could be diabetes (family history?). or something could be off. do this as a precaution. hope this helps.
Hi Sayonara ~ Jemma has many good suggestions. If he is also thirsty often, He could have blood sugar problems. ~ Wishing him good health, sleepy_angel
If he eats a lot, is fairly thin and is still feeling hungry, it may be a good idea to get his stools checked for any parasitic worms like tape worm.
The simple reason could be that he is not eating "filling" foods with high fibre and slow release energy.
Sometimes people do mistake their feelings of "hunger" when in fact the body is wanting fluids.
Tell him to have a healthy and balanced diet and cut out (or reduce) sugary, processed and highly refined foods. Nuts and meat (protein) is more filling and eating oats will also fill him up. He needs to eat slow release foods.
Best wishes.