What Do Presence Of Blood And Mucus In Stool Of A 4 Month Old Child Indicate?

Hi, my baby boy is 4 months old. Recently he had blood and mucus in his stool. I took him to the doc and he advised us to change his milk from nan optipro to neomino however this did not help. Apart from the blood and mucus in the stool, my son had diarrhoea. His poop also changed from yellow to green. I took him back the next day and he gave me antibiotics to give to baby to treat dysentery as his advised that baby picked up a bacterial infection. The problem is my baby is not satisfied with the new milk. I used to give him his milk with 1 scoop of rice cereal. He used to sleep between 2 and 3 hours at a go and his weight was fine. now he sleeps for only 45 mins to an hour at most. Can I add a scoop of cereal to the new milk or should I go back to his original milk?