Aggressive Cockerals

I use a decoy, a plastic bottle taped onto the end of a 4ft cane.
I've tried the holding, pinning to the ground, good talking to, etc. to no avail. I will try dunking him, but I suspect that the real solution is to get him more hens. Hopefully I'll have another half dozen hens in a few months time, but that's counting chickens before they're hatched

Be careful holding a foot up to him he could latch on with his neck and lacerate your ankles with his spurs! Some method of trimming spurs is vital if you keep a cockerel over a year.