What Causes Lower Abdominal Pain And Loss Of Appetite In Children?

Hi my son is almost 4 years old.. He goes back and forth between his mother and myself every other week.. He doesn t have a good eating schedule with her and eats a lot of junk food with her.. The past cpl of week that I have him he is complaining of lower belly pain and not wanting to eat.. When he uses the restroom it s coming out with a bad odor and it s sandy and almost like liquid.. It hurts him to wipe and he is getting a bad rash on his bottom no matter how quick I change him... He is straining holding on to tables and shaking trying to go but not much luck.. What should I do