[anigbrowl]

You could try a cognitive argument; beyond a certain number of hours/day, productivity/day starts to dive because you make more mistakes when you're tired and work at a slower pace overall. Failing that, try encouraging him to visit a therapist/psychiatrist. Maybe you could look into whether your company's health plan covers that sort of thing, often they do.

I feel your frustration. It's not really your problem, and it's irresponsible of his manager to let him wear himself down this way. His 'OCD' tendencies might well be an asset in the right context, like doing code review or QA or something instead of building code that's possibly meant to have a relatively short operating life.

[smt88]

I'm assuming that by "OCD tendencies", you do not mean a literal, clinical case of OCD. However, I think OP is describing OCD, the illness, not run-of-the-mill conscientiousness.

Despite the way it seems, OCD is not a rational illness, nor does it make you "better" at anything. It's purely emotional, and you can't reason with it. The sufferer feels a huge amount of fear/anxiety related to issues that seem insignificant to a typical person, and they can't choose which issues they are.

In fact, telling him that he makes more mistakes when he's more tired may give him greater anxiety, which would feed the vicious cycle of "doing more work to make sure I didn't screw up" that's going on in his mind.

[anigbrowl]

I qualified it because I can't really tell from the limited information the OP gave, is all; that's why I made a point of suggesting professional help. I have relatives with fairly severe OCD and have autistic spectrum disorders myself, but I'm also aware that self-professed medical conditions don't always line up with the DSM definitions so I take them with a pinch of salt.