The fact that he knows it is irrational is very good.

Study "mindfulness for OCD"

Also there is a book called "The mindfulness and acceptance workbook for depression" That is an excellent read.

He is definitely too attached to his own thoughts, he needs to learn to just view them and observe them without becoming tangled up in them. Of course that is easier said than done in the short-term, but through practice it will get much much better

S.E.E.
Sleep, Exercise, Eat Healthy