That temp is fine. While the butt might seem small, it will still take some time even at 250-275. I'm guessing that what you have is a partial/split butt. Perhaps 3 to 4 inches thick on it's shortest "side" ?
Here's a whole boston butt:
Let's say that it's about 12 inches long (left to right), 8 inches deep (front to back) and 4 inches thick. next, let's say that someone cut the butt in half right down the center from front to back. End result would be 2 pieces of butt, each one would be about 6 inches from left to right, still 8 inches front to back and still 4 inches thick. One of those two butts would take just as long to cook as the whole entire butt pictured above. Why ? Because the cook time for that butt is determined by the 4 inches of thickness.
Start the butt early. If it finishes faster than anticipated, let it cool a bit, wrap it in foil and hold it in a cooler. You can hold it like that for hours while the ribs are finishing up. It's better to have a butt finish early that to have a dinner ruined because the butt is taking too long.