I'd also draw a line around it in biro, note the time and then see how much it has spread in an hour (or how long it takes to spread a cm if it's faster!) - then ring and try to have a telephone appointment with a doctor.
If you saw them yesterday they should have told you what to expect - good and bad. They should be able to talk to you rather than needing an appointment if you are able to say so precisely how circumstances have changed. And then they can tell you what to do - whether you need to go to a&e (or at what point you do if it's not now), if you need stronger antibiotics and so on.
If they won't do a telephone consultation, might be worth ringing nhs direct or whatever it's called these days for a telephone consultation and they should be able to tell you what to do, including booking doctor's appointments.
Might also be worth putting together a quick grab bag to take with you in case your ds does need to go to a&e and stay in for a couple of days in case he needs IV antibiotics - pjs, toothbrush, phone charger, water/squash to drink, couple of snack bars in case food is bad, book to read, change for parking and coffee machines etc
If you don't need it - great. If you do - then you won't be running around doing it panicking you need to be going, you will have it ready to grab and go.
Hope your ds is feeling better very soon!