I Have A 2 Month Old Son He Has Problem

I have a 2 month old son he has problem with constipation. Well I don't know if to call it that because it's very loose when it dose come out. For his to have a bowel movement I have to mix prune apple juice in his milk. If I don't put it in his bottles he will not have a bowel movement. He was sent to a GI for his bowels and they want me to have him tested for hirschsprung disease. I'm very scared for him to get tested. I feel that the test will be very painful for him. I need advice