Recently, my male partner has complained of dizziness/seeing black spots towards the end of sex. This also occurrs whilst lifting weights at the gym. What could bethe reason?

Get him checked out. Though there may be a simple explanation for his symptoms, it would be smart to have him see a health care provider for a history and exam. Some cardiac (heart)-related problems can present with these symtoms. Always better to be safe, than sorry later on.
Needs to see a doc. Exertion related symptoms as you describe could mean partial blockage of blood flow to the brain.