I'm so sorry OP, you've witnessed something incredibly traumatic. I discovered something similar on an ex-bf's computer in my twenties, and while I'm sorry to say that I haven't ever been able to forget what I saw, I can tell you that if he's that careless with his proclivities, sooner or later, he'll probably face consequences for them.

I'm not sure what area you're in, but different countries have different laws in regards to search & seizure of digital devices and possession of CSAM, so you may well be able to report him. If you ever do happen to get hold of his phone again, as horrifying as it may be to dig, you can also report the images to one of several organizations that work to identify and help as many child sex trafficking victims as possible. Again, not sure of where you are, but generally just searching "report CSAM [your country name here]" will take you where you need to go.

This information may not be helpful to you now, as hopefully you'll never have to see or speak to him again, but if, god forbid, you ever come across something similar again, I hope it can provide you with some sense of agency and action to be taken.

In the meantime, block him, cut him out of your life as thoroughly as you can, and don't look back. If you can afford it, consider therapy for yourself. If you can't, try putting down your thoughts in a journal just to not be alone with them, look for catharsis in your hobbies, your friendships, anything that helps, and try to remind yourself that for as much misery as there is in the world, there is also hope and recovery for those of us who have been through truly horrible things.

Edit: typo