I Have A Six Year Old Boy That Keeps Having

I have a six year old boy that keeps having stomach issues. It's starts as constipation for three or four we give him meds his Dr told us to get but we usually have to give him an enema to get him to go real good. Then there's weeks when he has when he's so stopped up he will vomit for a day or two. And theses days where he will have diarrhea so bad he can't get to the bathroom fast enough he craps himself and he will do the six or seven times a day. We take him to his Dr but it's a never ending cycle. Please tell me what were doing wrong or what is could be? Is a serious problem? Any kind of information you can give us will be great. Thank you. God bless