He Just Started Punching The Tree

My boyfriend had gotten back from the store from getting me tampons. Me and him had a fight so he was already having a bad day. Something also happened at work so there was a lot of tension. He was sitting on the couch while I was cooking dinner and his mom called. I hear arguing and step into the living room to see what's going on. I don't like his mom for reasons. I have NO idea what she said to him, but it sent him over. He threw his phone at the wall and went outside and started punching a tree. I was going to try to stop him, but I figured it was best to let him get this out. He comes back in and he has tears running down his face and his hands are bleeding. He sits on the couch and I walk over to him and ask "Do you want to talk about it?" His whole face just breaks and tears start running down his face again. He shakes his head no. I lay down next to him and let him lay on my lap and cry it out. How long should I wait to ask again what happened. I just want to know what was said to set him off like that.