How To Cure Blisters On The Penis?

my boyfriend has recently found some ? ulcer looking spots on the shaft of his penis when he pulls his foreskin back. there are a few of these and they are about 5-6mm big. they are quite red and inflammed looking but not painful. does anyone know what these are ? is it a STI or STD. he is scared to go to the doctors but i really think he should. please can anyone help as we are quite worried