Son got bit by a Dog this weekend

Me and the kids went to my husband's aunt's house this saturday for her daughter's graduation party.
Anyways My son went upstairs to go play in his cousin's room to play with the toys(I had asked if this was ok with his aunt, she said it was fine) So I left him there to go and tend to my daughter. I came back to check on him 10 mins (if that) later to make sure he was doing ok. I heard him crying and screaming upstairs( i swear I never ran up stairs so fucking fast). By the time I got up there Kayla(the daughter)'s boyfriend had already had the dog off and away from my son. I went in the room and asked my son what happened and he said the dog bite him forever (back round my son has a language learning disability, so this is how he speaks). So I brought him down stairs to check out the bite. There was no breaking of the skin thank god but there is alot of bruising up and down his arm. I feel so terrible. I swear to god I would have never left him up there alone if I knew there was a dog up there on the 2nd floor.

My son knows that not all dogs are nice and to ask the owner for permission before even attempting to pet or touch an animal. We have been over this a million times.He has been around animals alot. But he got curious in a new enviroment (his aunt had recently moved into a new home)he went to check out the other rooms(probably on a mission for more toys) and found the dog. backround on the dog. The dog is mean natured and that is why is was put up in the first place. His aunt feels awful, I feel awful, My son's arm is all bruised up and I feel like the worst parent in the whole world :(

oh I had my mother check it out the next day (she is an RN) and she said he should be fine. I also asked his Aunt and the dog is all up to date on his shots. We iced his arm just incase of any swelling (i didnt see any but it was a precaution) we put magical bandaids and he was fine after that.