Streets had the same problem a couple of years ago, I *think* from diving into a bush, but who knows really.
I basically just ended up making him lay down and stay put until I could get the bleeding to stop, then literally stayed close enough to him for the rest of the night to stop any head shaking/scratching/rubbing and eventually a scab formed. It did take forever to heal though, and the cut was all of 2mm or something.
You could also try Liquid Bandage type stuff