What and how much have you been feeding him? Simply, 2% of the lamb's body weight, without exercise, is a holding ration. Though I doubt you actually want to hold him at 123 lbs but rather slow down his rate of gain. Therefore, I would put him on about 3.5 lbs of feed per day, plus a handful of hay with each feeding. This is going to slow him down a bit. Monitor his weight weekly and increase or decrease slightly as needed to get his average daily gain where it needs to be to reach your target weight at your show.
As far as exercising, sprinting is best.
