L.R.
Don't wait until the doctor is back in on Monday. Does your town (or a neighboring one) have "urgent care centers" also known as "doc in a box" medical offices? These are the places that have doctors and nurses but aren't hospital emergency rooms; they have weekend hours and take walk-in patients. You do not have to have been there previously for them to see you. These businesses have become more common even in smaller towns in recent years and we've certainly had to go to them when traveling over a weekend. Most insurance companies will cover visits there if your regular doctor is closed -- at least, ours will, and they're pretty fussy about what they cover.
If there is no urgent care center, does your pediatrician have any kind of "advice line" or a way to call the doctor to ask if this should mean a visit to the ER? Every decent pediatrician should have some form of after-hours advice phone service.
I wouldn't let it wait in case it is indeed an infection; it could spread and there are a lot of hours between now and Monday. If your son needs to start an antibiotic, you would lose two days before even getting him started on it, if you wait until Monday. Check out whether there are urgent care centers, call the doctor's weekend number, and/or go to the ER (last choice but it's there if you need it, and they do process kids faster than other patients).