You may consult a nutritionist and get a diet chart made which will provide you with adequate calories in
a healthy way. There can be many causes which can prevent
a person from gaining
weight. The common causes are hyperthyroidism, diabetes, severe anemia, certain STDs, worm infestation, and certain liver disorders. If you are unable to gain weight rule out conditions mentioned above. A slow and disciplined approach will help.Do keep me posted.
Take care and regards!