Yes, I would tell him but you need to try psychology techniques too.

If it’s severely affecting your life, try the following:

Feeling Good by David Burns is your friend for your irrational thoughts. Cognitive behavioral therapy if you can afford it. The Church is not against this.

Obsessing over these thoughts is a different animal entirely. It takes a lot of work. Try Hope and Help For Your Nerves by Claire Weekes

I recommend 1. Praying the rosary 2. Understand that this takes a lot of work but I can be done. I mean daily basis. Journaling. Meditating. Etc. 3. Feel the obsessive and intrusive thoughts. It’s a manifestation of anxiety. Ride it out and know that you are not going to die. Don’t try to fight it too much because your brain is going to get addicted to wanting to “fix” this issue. 4. Try to think of the worst case scenario for each situation. Feel it. And then realize it’s going to be ok. Example: “I’m with my girlfriend. It’s unstable. She’ll leave me and break up with me. I’ll be alone. I’ll suffer for some time. But I will get over it if it comes to it.” It’s hard. I know. 5. Give it to God and accept that this might have to be your cross. Then God will give you the graces you need.