Allergies particularly from AB's can come up later- for eg. you can take one course and have no reaction and then take them again at a later date and have a reaction. IYKWIM?

But if the course finished on monday and it's now saturday and he's got a rash, seems a bit odd- but i'm not sure what the half life is of penicillin (half life is how long it takes to get out of your system after you take it). But to be safe better to keep it in his medical history as a penicillin allergy, because secondary reactions are often worse. Sorry probably didn't answer your question.

HEather: an allergy will not necessarily come up straight away- some reactions can take quite awhile to develop. just depends on the drug/circumstances and the individual.