We've visited once or twice in early May -- most recently in 2018 -- and it was very warm, with daily highs in the mid-80s to 90, and overnight lows that didn't dip below the low 70s.
Perhaps the weather we experienced wasn't typical, but yours might not be either. There's really no way to plan this far ahead -- you'll just have to wait until a week or two before, when the weather forecasts are more likely to be accurate, and plan for the forecast (with some extra pieces to account for unexpectedly hot or cold weather).
Also, since you're from Boston, you'll want to take references to "cold" weather with a grain of salt. We're from upstate New York (Finger Lakes region), and at WDW, we find that we're most comfortable in clothing that's one step cooler (less bundled up) than the locals are wearing. (e.g., if they're wearing jeans, we're okay in shorts).