Do you have a command you give when he picks something up? I use a command every time my cockers pick something up or I give them something. When he's not sure what he should be doing you can use the command and re-assure him. Once he has learnt this on things he's happy with try it on fresh shot birds.
Try not to give him too many retrieves early on though, 2 or 3 out of 5, you get the rest, will keep him keen and help keep them steady.
Work on his drop to flush, he's less likely to peg'em then.
Try and see if it helps but don't rush him
Good luck