Simply saying "let it go", is probably more likely to pee the person off, to be honest, especially if it's pretty bad at the time. I'm not all that sure, if I'm honest - I'm so used to being on this end.

I'm really glad "he" (is this person a relative, friend, partner, something else?) found a way to mentally distract himself; that must've taken some willpower. I struggle to mentally distract myself, but it does happen from time to time, particularly if there's no other choice. Most of all, though, without my usual distractions, I tend to go loopy, and it's a really horrible experience. I use video games, music, guitar, German, and all that lot.

As far as new distractions go, I think what works best for me, is something that gets me thinking; something that distractions my ruminating/obsessing. A big help for me, is learning German, which I have, ironically, obsessed over, for about 4 or more years. Learning German forces my OCD to pee off elsewhere; it's brilliant. The guitar helps, because it allows me to use that creativity (OCD can suck that stuff up like a sponge) and aids in easing the anxiety, a bit. The person might wanna kick up a hobby; something that makes him think.

I think what might help him, is for you to help him be distracted. Help him to appreciate that it's just the OCD. By all means, be there for him; no need to abandon him. You being there, will help in distracting him. I don't know yours or his situation, but sometimes, due to the nature of my OCD, I prefer to be alone, to distract myself, and clear my mind. I get worse around people, sometimes; the guy you're referring to, might not have this problem, though.

If this person fears being a cat, and asks you all the time "Am I cat?" then he is seeking reassurance. What you'll want to do, is tell him something like "You know this is your OCD. You know this is irrational. You know you are anxious; that the anxiety is exasperating your OCD. You are obsessing - distract yourself." the more he obsesses, the more prominent it will be; if left to fester, he may even become convinced that he is a cat. It would help me for someone to say something like that to me. Unfortunately, we're not all worried that we may be a "cat", so it's probably not going to be as black 'n white as that. If this person is in CBT, it might be worth speaking to the therapist, and getting advice. I'll actually, if I remember, ask my psychologist, and see what she says.

I know the general gist is to learn to ignore the obsession, to accept the possibility that there is not always an absolution, and not to seek reassurance or do the compulsions. We have our own coping methods, such as our compulsions, but generally speaking, they aren't healthy, and only serve to strengthen the obsession, thus the cycle worsens.

I should also point out, if he were to - say - learn German, then there's a good chance he'll essentially substitute his compulsions (probably temporarily) for the German, which seems to be what I've done. :| Ultimately, he'll want treatment, as what I suggested (hobbies, learning something, an instrument, etc) will only act as a plaster for whatever wound that got him this way in the first place. I've been awake since 3-4am becuase I was tossing and turning all night, struggling to sleep properly; why? Well, all I remember is "dreaming" (doesn't feel like I was asleep enough to dream) in German, and I'm pretty damn sure I was obsessing in my barely-sleep. I stay up too late, studying, I give myself headaches, studying, and I cram probably too much of it into one day. If I'm away from my computer, there's a chance I'll be on my phone, looking German stuff up. If only I were doing a university course, but I've been teaching myself for all these years, so no gold star for me! It took me 3 or so years to get to the point when it became this obsessive. Eventually, I'll get so good at German, that I won't need to study this fervently, and I'll probably move on to something else - luckily, OCD will further grant me with great language skills! ¬_¬ Oh, I also obsess over English, which is probably why I look like a huge nerd, right now. Point is, I'm being more productive with how I cope with my OCD; when you have nothing else, this is at least something.

If you have any other questions, I'm happy to give it a shot. I'm not all-knowing, but if there's one thing with which I have experience, it's OCD!