My 5 year old has a tooth abscess and his dentist can't see him until monday. Should I try and find a doctor to start antibiotics before then?

Call your dentist . I am sure if you talk to your dentist he will respond to your need.
Tooth pain. If there is a recent medical history on record at your child's dentist's office, they should be able to call a prescription in to the pharmacy, if you've spoken with them, and they think the symptoms warrant an antibiotic. You shouldn't need to find a different doctor who can call the pharmacy for you.
Your decision, but.. Anyone who has any infection should be on antibiotics as soon as possible to limit the severity and prevent spread of an infection, regardless of when definitive treatment will be performed. If what i hear is true, in my opinion, you have two reasons not to use this dentist. First, your child should be seen sooner, and secondly he\she should be on antibiotics. Find another dentist now.
Call in. If your child has been with his dentist, he or she can prescribed the medicine over the phone as an emergency measure and see the patient at the earliest appointment. Abscess tooth hurts and the prescribed antibiotics could help the child within 24 hours. Call into your dentist and request for antibiotics as an emergency measure. Abscess tooth can be very painful!