How are you training him as a sheepdog?
Are you actually training him to sheep, and do you own the sheep?
I am a little puzzled as to how you stop (down) him when you are working him? Or is he deaf to your commands then as well?
Chasing everything is what collies do. I have seen a working farm dog be out 4 hours rounding up year old cows and transporting them via roads to another field - we got back to the farm and the dog immediately started rounding up the chickens in the farmyard!!!
This same dog would round up anything - if the cows were in the milking yard he would go outside the gate and circle non stop until the cows went into the milking parlour.
Some dogs are just bred to work.