It's going to take a good piece of luck to make your minimum weight, but most luck comes from hard work and following a good plan.
The 3 things that will help a lamb be heavier is water retention, full belly, and don't burn off any more energy than you have to. Already some great tips here, and I'll add some ideas to hopefully get this guy and his showman into the lightweight class.
I would not exercise him beyond showmanship practice. Extra running for muscle tone will also burn off fat, and you need all you can get right now.
I would not wash or rinse him for 24 hours prior to weigh in, since a cold shower will usually make any animal pee, and that can drop his weight enough to slip back under your minimum.
Electrolytes will help keep his muscles hyrdated, adding water weight. If you can find some Propylene Glycol, add about 8 CC's to the electrolyte. This is an energy source, and gives their throat a feeling of "I need a drink to wash this down". Don't expect him to just run over and guzzle water, he will get it when he needs it. Another thing to add to the electrolytes is Creatine, a muscle building substance found in the bloodstream. Walmart pharmacy usually has it in some form, in the Protein shake area. 1 tablespoon, mixed in with 30 CC's of electrolytes/8 CC's Propylene Glycol, 3 times a day.
Speaking of water, do everything you can to bring your own from home, and use the same bucket that he drinks from at home. If water tastes different, he might refuse to drink, which is not good.
Lastly, keep him full. Stay on your regular feeding schedule, but an hour before weigh in time, give him anything that he wants. Alfalfa is a great filler, and might give you just enough push to get over the limit. Straight corn is good too, but mix it with his regular feed, as too much by itself can upset his rumen.