My boyfriend wouldnt masturbate in front of me. He says it pains when he has an erection?

Interesting story. Pain with an erection may be due to a tight foreskin or penile frenum (fold between glans and foreskin), chordee or if too much skin was removed at circumcision. He may also be too embarassed to masturbate in front of you. He would also want to avoid sex if erection is painful. Perhaps he should go for a consultation with a urologist.