"You're Nuts!" is not socially acceptable, even when it's 100% true. We probably all have a friend, and definitely all have an in-law, who is in fact certifiably nuts. Sure, he could be a Big Wig, head of a studio, President of Some Big Thing, but we all know he's N-V-T-S nuts. But how do we tell him that tactfully?
We'd suggest you do it with subtle hints. A squirrel. A chipmunk. Work you way up to the nut bowl. Maybe he gets the hint. Probably not -- nutty people aren't good listeners. But even if he doesn't, at least you could inwardly smirk at your cleverness. And share it with someone else who's not nuts. Or at least the same nuts as you.