Hi well come to HCM.
You have ot mentioned his age,any other complain like fever
is very essential to know before giving your answer.If nothing of this short involve him
in some exercise like walking,swimming, wt lifting etc.Extra physical activity will definetely
make him feel hungry.Is he fond of more of out side food than home made find out
that and try to improve that and train him for good life style for sleep, food and exercise.
If he has habit of taking excess tea coffe,cold beverages,tobacco,smoking
be corrected grdual counselleng over a period of time he can improve his appetite. Thanks