My bf of 3 years is cheating?

Throw away account and on mobile. Sorry foe any bad formatting.My boyfriend and I have been together for 3 years now. It's been an amazing relationship. He's unlike any other guy I've dated before.However, recently I've begun to notice some changes. He seems more short tempered with me, he hasn't been as affectionate, and most concerning is that he seems to be hiding his phone from me. These are all thing I've noticed over the past couple months. Naturally, my concern has been growing.Last night, he was over at my house doing homework. He came to bed late, plugged his phone in to charge, and went to sleep. About 20 minutes later, his phone buzzed. Since the phone chargers are on my side of the bed, I reached over to check his phone. On the screen was a message notification from Scruff. The message itself was innocent and I didn't click into it. Needless to say, I had no idea he had the Scruff app on his phoneBy this point, I'm wide awake. My heart is pounding. I'm laying next to someone that I love deeply, but now I have very serious concerns about our relationship.I went into the other room and quickly downloaded the app to my phone. I found his profile easily. It said that he's looking for friends and that he goes on there whenever he is bored. Afterwards, I uninstalled the app from my own phone. I couldn't sleep the rest of the night and just watched Netflix in the other room until it was time to get ready for work. I woke him up to tell him I was leaving and that id talk to him later.Obviously I feel very betrayed. Up until this point, our relationship has been built upon trust and now that trust is shattered.I don't know what to do from this point. Do I ask him about it directly? How would I even start such a conversation? Do I need to gather more evidence? Anyone have any advice?Sorry for the long post. I doubt this will get any replies. But it helped to just get it out there.Thanks all!