What Causes Blood Blisters On Penis Head?

Father (84 yrs old) has what looks like a couple blood blister on penis head. He has not been circumisized and does not have (or ever had) and STDs. It is swollen and has one blister that has not popped yet. Trying to understand what the cause is and what type of treatment we should get him. Can you suggest what it could be from and what treatment should be?