He stocked his pond only a year ago. I would say a cross is not logical. The fish haven't had time to grow up, mature and then spawn to create a hybrid that could be this size. If this is a hybrid, it had to come from the hatchery and stocked as a 3-4 inch fish. Mouth size and all other markings suggest pumpkinseed to me. I agree, the gill tab should have some red, but I have seen much stranger things.
Just for clarity...I have been known to be wrong way too much to suit my ego.
I just can't see how it could be anything but.
My guess is that this fish was common to the watershed and has grown to its size. I would just enjoy it and not worry about it. It's not a big deal.

Teach a man to grow fish...
He can teach to catch fish...