Suggestions on constipation for a 13month old?

My son was on formula before and ever since i switched him to cows milk the whole one he gets really constipated, ive tried contacting his doctor and she never returned my call im going to try an switch him to a pediatrician by this week but i really don't know what to do anymore when ever he goes poop his rectum stretches an he poops big hard stools and its painful for him ive tried juice, prune juice, he drinks plenty of water and eats food all day tru out the whole day he has about 14Oz in milk he loves fresh fruits an veggies any suggestions would be a big help im a first time mom so idon'tt know much thank you for your time=)