Hi Ski, lovely looking dog mate.
Normally dogs 'learn' bite limitation when with their siblings, all part of the rough and tumble games they play.
If, as you say, he's biting and drawing blood you have to get on top of this immediately, a very firm, loud NO would be the first ploy.
If that doesn't work I would use a rattle bottle, a small plastic fizz bottle with a few stones in the bottom. Keep it hidden, by your side, in a pocket whatever, he doesn't ever need to see it or hear it untill you use it. Again, a loud NO with a hard shake of the bottle at EXACTLY the same time.
Basically you are trying to distract him from the behaviour you don't want and introducing a command, timing is crucial when correcting behaviour with this and many other methods, a second too late and your'e wasting your time.
We had to euthanise our old rottie, almost 13 years old just before christmas. I've had rotties for over 20 years and couldn't live with out one. As we always rescue we have just rehomed this 9 month old fella..........
Good luck with the training mate, give us a shout if you want.