Is there any supplement I can buy to add to my dogs food to make his coat looking shinny? He's a black collie grey x bull grey.he's on a very varied diet. Beef off cuts from my local butcher,pigs heads,rabbit,all with added veg and mixer biscuit.hes always wormed to date he's got a good build on him it's his coat that's lets him down.i'd like to show him this summer so any advice much appreciated thanks