The only thing that will get rid of an abcess completely is either having a root canal done or having the tooth removed, anti-biotics hold it off for a little while, but it will keep recurring until he goes to the dentist.  Not only that, but check out this article  by mum2four and if at any stage he cannot swallow or has trouble breathing call 000 as there is always the chance of something called Ludwig's Angina as well.  So please please make the trip to the dentist a case of sooner rather than later!

In the meantime, Icepacks on the outside of the mouth, 20 mins on 20 off, and as stupid as this sounds, a cold used teabag on the gum works a treat too!

Gah, the link for ludwigs angina didnt work, try this instead  http://en.wikipedia.org/wiki/Ludwig's_angina