Pale gums indicate internal bleeding, and that's confirmed with the blood clots he's passing. What could be causing the internal bleeding? Hard to say. You may want to call your vet, just in case this is a virus that could affect other calves.
As for point of no return, that's hard to pinpoint. I've seen some practially come back from the dead, and I've seen others that didn't look so bad just drop dead. If he's still up and wanting to nurse, that's a good thing. If he quits wanting to nurse, and he's getting too dehydrated, or is down, then it's IV time.
I'd take him off milk completely and switch him to just electrolytes. They can go a long time on those. You'd be surprised. Whatever's irritated his gut enough to make it bleed will be made worse with milk. The electrolytes will give it a chance to start healing. You could try some kaopectate to coat the gut. That may help some. It comes in a veterinary form.
But I'd still call the vet. You could take a stool sample in, and they could test it for viruses and cocci.