... make him sterile.
The alternative would be to cast an older actor and have the character die of old age, as in the books.
The show, however, went for Ricky developing, uh, space cancer and being zapped sterile by his alien BFF ... just so they could cast a hunky young actor, give him a love triangle and have a shorter timeline between the alien's first appearance and Earth's destruction.
have a shorter timeline between the alien's first appearance and Earth's destruction.
Although I don't agree with shortening the time frame, I can see that it made more sense from a TV adaptation viewpoint. None of the human characters in the early part of the book are still around even in the second half, let alone the end, so you'd have the problem of dealing with two separate (and unconnected) sets of characters if they'd stuck to the original time frame. The point was that Clarke was more concerned with the overall concept, less so with the characters involved, but the TV version approached it the other way round. Horses for courses, I suppose...
Was the "love triangle" (his old wife that died apparently from bone marrow cancer) in the book or added for the TV show?
It's definitely soapy and makes the emotional manipulation pretty clear.