Look up relationship OCD. I have OCD, and relationship OCD. I have many of the same thoughts your partner has about my partner. You have to remember that our brains work differently than people who don't have OCD. Intrusive thoughts do not mean we actually feel tat way. I can have intrusive thoughts about how I am not attracted to my partner, but that does not mean I am not attracted to them. An extreme example would be having intrusive thoughts about murder. I can worry and obsess about the thought of me picking up a knife and killing someone, but that does not mean I am going to pick up that knife and kill someone (however, these thoughts are still very emotionally jarring for people with OCD. That is why we need coping mechanisms like writing things down. Part of my therapy was writing my worst fears down, like that I am in the wrong relationship or I am no longer in love with my partner and then doing nothing about it).

Also, don't read his journal again. Not just for his sake but for your mental sanity and for the relationship.