My Boyfriend Doesn't Cum!

Okay so I can tell my boyfriend enjoys me and I soo enjoy him but he like never cums when we have sex! We could have sex three times a day and he just won't cum. 
In the past we talked about it and he has always told me 'it's not you, so don't feel bad... I can just last a long time' 
Anyways it makes me so nervous. 
Help please!