My baby have a heart mummer?

i am ashley and my baby haves a heart mummer and he is 8 monthes old and he has had it seen he was born he gose to the dr thurseday and i am scared because the dr said that the mummer is a hole in his heart and he has a van that anit getting alot of blood to it and that is why the heart mummer is causeing him to lose blood from his heart he is a nemce now because he is loseing blood somewhere i hope everything will be ok when he was born he lost alot of wight thay just told me that he had a mummer when he was 3monthes but that new about it but thay thought it would close up but it did not close up so we are going to dallas to see what thay are going to do now i know i should not be scared but i am i just want to know what cause a heart mummer and i want to know what i need to watch for the dr dont tell me alot he said it will be ok but my baby is 8 monthes and he is 20 pounds but he dont look like it so if anyone can talk to me and tell me what i need to watch for.what do you think i should do