AIO for Boyfriend reaching out to Ex about Our issues? (2yr)

Hello, new here so if I do something wrong, please tell me.

My current boyfriend (M21) and I (M20) have had our issues for some time, but we always pull through it at the end of the day. Today is Thursday, May 15, and when I saw a concert in Camden, NJ on Sunday, May 10, I came home around lunch time, took a 6hour nap, then woke up and called my little bother to talk for some time, then I crashed again, not really communicating to my bf when I got home from being so tired… Apparently that Sunday night, he was so upset that I had not talked to him, and the fact we’ve been having issues lately (arguments over stupid little stuff), he reached out to his ex-bf and just wanted to talk (on Snapchat), for comfort. Now, throughout this week we finished our last classes at college, went to Applebee’s a few times, went to the gym together, had intimate moments and he never brought this up at all, and showed no signs of guilt for reaching out to his ex throughout the week. Today, Thursday, he tells me about him reaching out to his ex. I, rightfully so, got angry that he didn’t tell me Monday morning. In my mind, it’s sneaky, he never told me, he’s been hiding the truth, and showing no signs of guilt. He claims that he has no one to talk to so he said it’d be best to reach out to his ex. This is completely unacceptable right? I’m angry, pissed off, I have no words for my bf right now. He crossed a boundary and the least he should do is apologize. I can’t come up with a reason why to understand his point, and nonetheless forgive him. So, AIO? Any help/support/advice is what I really need right now. Thank you.