Why dosnt my boyfriend cum when were having sex?

Partner problem. I'm not certain that you are describing a failure to ejaculate or ejaculation without semen. The first can be due to diabetes, psychologic stress, hormonal issues or vascular problems. Small semen volume can be related to retrograde ejaculation when due to surgery, nerve damage or diabetes the sperm ejaculates into the bladder rather than exiting the penis.
Men vary. What makes ejaculation happen or not happen varies tremendously from man to man. If he ejaculates like other men under other conditions, you can work as a couple to explore whatever it is that will leave both of you satisfied. With so many people of all ages taking ssri's nowadays, this may be a factor. Solving this problem will bring you closer together as a loving couple. Wish you both the best.