Do you happen to know if he abuses pain pills or takes blood pressure medicine? Also, alcohol (if before sex, drunk, etc) tends to prohibit or delay ejaculation. If he's watching a lot of porn that you are unaware of, and masturbating, it eventually makes them unable to function normally. If this is the case, research this topic on the Internet. With today's technology, porn is just a click away. There will be so many young men and boys that will be affected and their future partners will never know, but they will blame themselves and be in sex less relationships.
Whatever your case may be, please know that it is NOT YOUR FAULT!