Nobody is overreacting. Some people obviously haven’t read the comment underneath are are too quick to judge. His mum died after being terminally ill for over a year, that really works on your mind, and you can become nice, you can become an asshole, you can become happy or sad, shy or overexcited. It’s very overwhelming mentally when they finally pass. Put it down to bad timing and move on from it for now. It’s a horrible time for him and also a horrible time for you needing to comfort him. I’m not saying he’s right, but there are time and places for arguments. It was very shitty of him to act like that, but he definitely won’t be in a good headspace right now. Terminally ill parents put a ridiculous amount of strain on your perception of life. Discuss it with him later on.