Is this abuse or am I out of line?

My son has been at his dad's house for several days. He came home last night and immediately started telling me how he had the worst ever. He said he was hit over and over and over again and then didn't stop crying so his dad started hitting him again. Needless to say, I was outraged. I asked to see his bottom so I could check for marks and sure enough his bottom has bruising on it STILL and this incident took place Sunday. I have made an appointment to see the pediatrician this afternoon so they can tell me what they think or what should be done. Am I right in my thinking that this has gone too far or am I just out of line? My son and daughter have complained several times about their dad giving "real real bad ings." I've just never checked their backside for bruising before. Four days just seems like a long time to still bear marks from a .