My boyfriend fell on his bike at a young age and hit his testicles so hard that he said he urinated blood. could this cause infertility?

Possibly ...: Testicular injuries may or may not cause fertility problems in later life. If you're not trying to get pregnant assume he's 100% fertile. If you're planning to conceive he should get a full semen analysis with antisperm antibody testing to make sure that he doesn't have a low sperm count, or proteins on his sperm that can prevent them from fertilizing an egg - injury is a risk factor for these.