Thank you for your query.
For a premmie I am happy with the weight of 10.7 Kg at this age. Yes, there is scope for improvement but one should not push it unduly. One should ensure that the child has a well balanced diet which shall provide all the essential nutrients. No particular vitamin and calcium supplement
shall help in increasing the weight. You can provide him with a multi-vitamin and mineral supplement to supplement any vitamin and mineral deficiency
Just to repeat esnure that your son has a well balanced diet containing fruits, green vegetables, cereals, pulses, milk and its products like curd, paneer and in non-vegetarian families, meat and poultry, in adequate amounts.
I hope I have answered your query to your satisfaction.