What Causes Bleeding From The Rectum When Having A Bowel Movement?

My child (4 year old boy) is having bleeding from the rectum every time he has a bowel movement, has been happening for about 2 - 3 weeks. His anal area is also red and he is swollen, I believe it is his perenium that is red and swollen. We went to a doctor who told us to put vaseline on it so that it would heal but so far that is not doing anything. I am getting desperate to figure out what to do since he is not trying to avoid having bowel movements because it is so painful and cries a lot when he does go.