2yr old Toddler not eating food. Food!

My child is 1yr10m old, for the last 6-7 months he is not eating food everyday due to which he seems weak/lean. He eats food once in a week or two that too in very little quantity, we keep offering him food that contains sweet/salty oats {daliya}, sweet/salty noodles {sewai}, rice, chapati {wheat/corn flour}, eggs but he does not open his mouth to eat even a single spoon. He is having constipation problem also and passes stool almost after 4-5 days that too in small quantities, we daily feed him "Laxopeg granules" (prescribed by doctor) so that stool remains soft but still he takes so many days to pass stool. We give him milk during night {3-4} times} and only once during day. He is more interested in eating chips, biscuits and this kind of junk/fried food which we mostly avoid. He eats a lot of sweet items. We are unable to feed him chapati/rice/daliya etc. as he does not eat any of these. Any solution/suggestin as to what could be done so that the kid starts eating food