Acidentally found something I didn't want to see..

My boyfriend was messing around on my phone earlier in the day.. well when he left I clicked on my yahoo app to check my mail and he was still logged in. Naturally, I scrolled and found a few emails that caught my attention... he's been responding to sex ads on Craigslist :( I'm not 100% sure if he's followed through but I need advice, input, anything really. I'm just not sure how to go about this.