Thanks for your query
Your boyfriend:s one sided vasectomy has been in childhood for some injury,and now he has noticed pain during ejaculation
.In fact his pain is not related to the vasectomy at all.as vas do not play any role in ejaculation .He might have a chronic prostatitis
Pl consult Urologist
who will do per rectal examination
to confirm this.Pl get your Ultrasound scan done and take appropriate treatment.