Others with more experience may be able to give more advice, but what i can say is simply that you need to tell him as soon as possible. There's no good time to break up with someone, it's always gonna be at least a little rough when it's a one-sided breakup
If this is the third time he's asked and it only worked out for a few months, then i have to say he probably won't be surprised unfortunately 😭 Pick a time, be honest and just tell him that you made a mistake. You thought you were going to be happy in a relationship with him but you were wrong, you're definitely aromantic and won't be happy in a romantic relationship (not to say that you can't be happy in a relationship, but that's probably what you should tell him lol)
Yeah in general it seems you need a break asap and he needs to be more accepting of support. In the future you'll be better able to set boundaries so you won't be in this situation again hopefully!! best of luck to you, i'm here to talk if you ever need to!!