gomexjr, think you need to go and think about probability some more - and stay away from the roulette wheel! You did make me chuckle
. Following on from Scattercat's comments, according to your logic if I buy a lottery ticket I've a 50/50 chance of winning - 'cos I either win or I don't. Now that's a classic.
SASless, think about the 50% as being a 50/50 chance that it will rain in the forecast area over the duration of the forecast, scattered throughout the area in his terms. Just tell the boss it's an evens chance, but you don't have a crystal ball - if you did, you'd be in Vegas earning a fortune.
So da boss has a choice just like putting the welfare of the warbirds on one toss of a coin or spin of the roulette wheel on lets say black. You've got to make him understand that for a start. Nothing you can do to predict further than that.
However, let him know that as a highly skilled aviator you can get the odds slightly more in his favour by going up and having a look on the day - maybe you can dodge the rain as others have suggested. But if you've got a line in front of you, then just tell him you'll be turning for home if he doesn't want his planes to get wet.
Bottom line that he's got to understand is that there is a risk. I'd say about a 40% chance of his planes getting wet, given the 50% chance of rain, but with you weaving about the sky to avoid. Ball should be back in his court - does he want to take the risk. You've explained it to him, shouldn't be up to you to decide.