You don't need to worry about cooling too much. Just drop it by, say, 2 degrees. Doesn't sound like much to us, but it makes a big difference to the snake. And, yes, I'd wait until he's digested this current meal. Then when you feed again, he'll be used to it. It's not that big of a deal for a non-breeding animal. You could drop the temp for the next 4 weeks, or not at all this year. Since he's a touch overweight (very slightly), I probably would go ahead and do it. He'll burn up some of his extra calories that way. They don't use their fat to stay warm like mammals do, but the cooler temps will still help him work off some of that weight a little.