Hello, It's almost normal as a baby boy of 3 months weigh upto 6 kg. I being a homoeopath can suggest you homoeopathic recourse for your baby:
30-1 drops with 1tsfl of milk.
Your second daughter be given protein
, eggs, Mangoes, almonds
to gain weight with the same Homoeo medication dose be 3 drops Tk, care.