Most GP's would not mess around with Tonsillitis and give a Penicillin AB to take straight away.
Some fevers are good fevers as in fighting the virus, Tonsillitis is a different kettle of fish. It sounds like he has a pretty mighty dose of Tonsillitis so I would give the AB's another 12hrs to kick in before going back to the GP.
My niece just had it, her AB's kicked in over night.
Just make sure he is getting plenty of fluids, don't to be worried about not eating as he probably does not feel like it and it might be hurting his throat. Iceypoles may help.