I guess hopefully is just his baby teeth that are rotten and the underlying teeth that replaces them will be OK. But I think you need to address what he eats and his tooth care now at 6 he is not old enough to care for his own teeth and you should be doing the brushing especially the difficult to reach back teeth (there was a recent program on child tooth decay in the UK). Get him drinking water too so his mouth is not subjected to sugary drinks.
Hopefully the dentist well have lots of practical advice for you on children's tooth care and what to do with the tooth that is hurting your son now.
Good luck.