How To Increase Weight In Toddlers?

Hi, i have a 2 months son. His birth weight was 2.740 kgs. On discharge it was 2.5 kgs. N since then till 40 days it remained the Same. After 40 days started gaining slowly but real slow. Only reached hia birth weight. He was on bf till last two weeks. Now since two weeks included formula feed twice a day. He is being fivwn given d3 drops n iron supplement. What else can b done to help him gain good weight.he will b 2 months on 26 mar. Regards