Hope you were able to get him to the dentist this week. If so, what did he/she determine it to be?
My 6 year old daughter had an abcess as well and it turned out that she had a very bad cavity in one of her lower baby teeth but she didn't complain of any major pain. The original filling had fallen out which left that particular tooth more exposed to decay. The dentist pulled that baby tooth since it wasn't worth it to put a big filling on a baby tooth. The abcess developed near that tooth, on the gum line, due to that big cavity. When the dentist pulled the tooth, the abcess opened and drained. He said that antibiotics were not necessary since the mouth heals very quickly. That same daughter also had another small abcess on the upper gum, near one of her canine teeth. There was some internal decay occuring on that tooth so my dentist did a pulpectomy (it is like a baby root canal but doesn't go deep into the roots) - basically he cleaned out the internal pulp and filled it with whatever they used. The abcess went away on its own.
Every case / infection is different so please don't panic until you see the dentist.
Hope this helps!