Hi and welcome to HCM. Thank you for your query.
If there is no bruising with no trauma or some specific cause then he should do hematolgoical tests to see is there clotting abnormality or platelet count
disorders. In most cases this is nothing serious, but you should do these tests to rule out some blood diseases.Also it can be allergic reaction
and it should subside in several days in this case.
Wish you good health,regards.