Thank you for the question
Well different people have different metabolism . His being thin is not a big issue per se but taking many capsule of revital is a problem. He might end up with hypervitaminosis. Hence he needs to stop or reduce it . My suggestion is to stop it.
His body type might be ectomorphic hence he doesn't gain weight easily . Not to worry .
Next Steps
1)he can start taking high calorie diet .Horlicks or other weight gainers is a good option with nuts , bananas etc.
2)he doesn't need revital.
3) join a gym and get a proper trainer .
4) check his hemoglobin levels and deworm him( tab albendazole 400mg stat and then after 10 days again )
thank you.