What Causes Frequent Urination And Severe Constipation In An Infant?

My baby is 18 days old. Since last 15 days he had passed stool only 3 times. He also has gastric problems. I am giving him neo peptine 8 drops twice a day even then there is no improvement. He urinates frequently please suggest what should I do to hv his motions regular. He gets restless because of the gastric problem and he vomits occasionally