I just found the owners manual and here's how the clutch is supposed to be adjusted on these ATVs.

 

Then just try an 1/8 turn clockwise after you feel resistance and see if it shifts better. If this adjustment doesn't help or even a tad more clockwise or even counter clockwise like most machines adjust to,then you may need new clutch plates,plus check out the centrifugal clutch while you're at it.Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse

 

He drove it home after I adjusted it last night and he said it's shifting great again, better than it has in a long time.