Thanks to all. To clarify he has no trouble with hold when walking on a leash or at heel or jumping up onto a platform or table. It is just when I say sit he drops it. He definitely knows what sit means and as soon as I say it his butt hits the ground but he drops the roller. If I put it in his mouth while sitting the butt comes up but a correction (tap on the butt) or saying sit puts the butt right back down but he drops the roller.
Picked up two ducks today and brought them 30 or so yards through the water across a few yards of land and jumped into the blind without dropping them. I just didn't ask him to sit .
I think I'll just start force fetch and go from there.