Vaccination

My son is 7 now.
A year and a half ago my son got purple patches on his skin. He had some stomachache and one of his ankles was swallen. The doctor suspected a mild condition of HS Purpura.Everything cleared in a couple of days.
We did some blood tests, urine test and stool tests. They all came clean.
We saw an immunollogist who said that she thinks it's not HP purpura because everything cleared so quicklly. We were sent home.

From time to time he still has one or two purple patches but no other symptoms.
He is supposed to get MMR vaccination next month.
Is it safe to vaccinate since this is so called live vaccination?
His HP purpura was never officially confirmed.