If he's nicking the inside of his pasterns and on his fetlock - neoprene yorkshire boots are brilliant - they protect from the coronet up to the joint. Or if it's just the coronet you can get your saddler to make you up something like this - just fasten with a velcro strap. Again work well - seems to be a common place for cuts/nicks on flat horses. https://www.equipro.com/Scalping_Boots.html