He's probably very young. To teach him what to eat give him some grain, animal, mushroom, etc. and instantly rub his belly while it's in his hand and he'll eat it. Praise him after that and he'll learn to eat that more, plus click the sea to drink.
Also, for sleeping at night, click the center of the creature pen and he will eventually sleep there. And the fatness is not a problem, he will grow normally, but if you don't like it, give him some toys (Teddy bear, beach ball, dice, Bowling ball) and in his free time he will play with them and get much stronger and work off the fat,
Hope it helps.
