Erosion is slightly different for each individual being. But the biggest safeguards would be to keep him well connected to his humanity - through loved ones or through a sense of duty or purpose.

And keeping him away from those beings that have already succumbed to the darkness. And definitely away from the Abyss itself.

There have been a few cases of humans granting longevity - sometimes intentionally and sometimes through a curse - though they are few and far between. I can research such matters further.