MMR and antibiotics

Patient: Today at the nursery I have been given a letter than my son, 7 months old, needs to have the MMR vaccine in the next 48 hours as has been in contact with another child with measles.the main concern is that he is on antibiotics, clarithromycin as he has been with a very bad cold, so the gp said that he can not get the vaccine.can you please advise on the risk if he is not vaccinated or as he is on antibiotics he is safe, I am really worry.