My boyfriends left testi is split from a bike wreck, they did an ultra sound and they said it was okay. but could this effect his fertility?

Possible: Hopefully not; possibly yes. Depends on the extent of damage and the amount of bleeding involved; mainly, it depends on things you can't measure at this point. Sometime after healing he can be evaluated with a semen analysis.

Likely scrotum split: Split testis would be seen on ultrasound & bf would have been advised to have it repaired in operating room under anesthesia. Split in scrotum much more common . Might have been sutured in er, or left to heal on it'ss own. Fertility should be unaffected. Hope he was wearing a helmet!