Undecided Question

So we’ve been together about a year and two months. I know he’s always had the lower tolerance out of the two of us- I’m really mellow and easygoing and stuff gets under his skin and he reacts differently. I don’t care when he gets upset about things, situations etc…but I don’t like when he gets upset with ME.
It seems that early on obviously, we never had arguments. Now, it seems if I bring something up I’m not pleased with, he’ll turn it into a fight instead of something constructive. Or like this morning- he BLEW up at me, started yelling because I changed my mind about getting coffee- I blatantly refused and said ‘not this morning’ because I don’t want him to be late to work [he drops me off before his job] and he got mad! I wasn’t preventing him from making a decision, I was simply not in the mood by the time we got out the door- it would have made him late and I would have felt bad- even if he didn’t.
It seems the stupidest things set him off. The way he would react to little things I would expect the type of reaction for some serious offense. He says he loves me a lot, and I know he does. But it’s hard picturing myself happily married to him like we talked about before when he can’t even stay civil with me in our conversations. He is being SO immature.
What should I do? Talking about stuff makes him mad!
Thanks.