For a 15 month old to weigh 7.6 kgs is certainly less than expected. He should have been over 10 kg by now. Also a length of 75 cms corresponds to that of a 1 year old.
His birth weight
was only 2.2 kg. Was he born after a complete term of 9 months or pre term ? I am asking this question as his catch up growth would vary in that case.
He should be given calorie dense foods like bread with butter, biscuits, cereals and eggs as part of his daily routine.
Also ensure he has access to clean and distilled water all the time and does not consume outside food. This can help in reducing the incidence of gut infection.