Need details to prescribe medicines.
Next Steps
Avoid bitter gourd or any bitter astringent diet in his food.. As they worsen the condition.
Health Tips
You can add generous amount of ghee and milk in his diet. Because both are the best aphrodisiac foods... Can use daily lifelong. Only precaution is ghee and milk should be warm enough to consume and should be along with fresh hot food. For better absorption hot water can be given to drink.
If he cooperates application of warm sesame oil to whole body just before bath by own or by you will increase stamina and general health. Calms mind and affects on vitality.