Two great ones will be:

Honey Springs - Sept 20-21
Prairie Grove AR - Dec 6-7

Yes I know you said summer, but these are within his range.

The only thing I know is happening this summer is a living history at Perryville/McAlester. There will not be a battle this year. It is just too hot in August.

The Battle of Westport in Kansas City will be May 17-18, but that would be about a 4.5 hour drive from Stillwater.