Constipated for 3 months!?

Okay. My son is 14 months old, I put him on whole milk (well started transfering him ) at 11 1/2 months old. Since then hes been constipated, the doctor said to try prunes, and juices, water, karo syrup. nothing worked, so 2 weeks ago i call her and say look hes still constipated, he screams bloody murder everytime he tries to poop and hes doing it for 20 minutes for bearly nothing, so she told me to lower the amount of milk i give him from 20 oz to 8 oz a day, so i did that but hes not wanting to eat foods, he slaps it away so i wait for a while figuring hell eat when hes hunry and he just gets cranky and screams until a bottle. Anyways, so i cant get him to eat foods, So i put him back on Formula the Similac Go and Grow ..that should be good hu? 3 days after i gave it to him he pooped and it was good. so when should i try to transfer back to whole milk?