As time is of the essence, if it has been <1 hr since he ate this, consider inducing vomiting or giving activated charcoal but he is going to need to see his vet as soon as they are open. The good news is that this is an anticoagulant toxin (not a neurotoxin) so the adverse signs are not instant. They are gradual over days b/c the poison works by blocking the production of clotting factors. So, there are no signs til the body uses up the ones already in circulation. Once they are used up, we see bleeding both externally (from the mouth, anus, etc) and internally (which can affect breathing, cause weakness, pale gums, distend the belly with blood).
So, what we need to do is to keep things calm and keep your wee one rested. To induce vomiting, you can use either washing soda crystals (NOT CAUSTIC SODA!) or 3% hydrogen peroxide (1ml per pound of his weight) to induce vomiting. For the soda crystals, we usually take a handful and add a bit of water to make it into a paste that we can roll into a ball. Once we do that, we need to put it right down the throat. After use one hand keeps his mouth shut and the other hand to stroke his neck to get him to swallow. Vomiting occurs within several minutes. If he brings them all up, we can breathe a sigh of relief but I would still have him seen so that they can dispense further treatment against any residual or absorbed toxin.
Also I would note that most practices here have hours today or at least have contingency plans for emergency care for their patients even when they are closed. Therefore, it is worth ringing the practice. If they are open, you can get in today. If they aren't, then they will likely have a message to direct you on contacting their out of hours service. And if you don't have a vet, you can find a local one via the RCVS Register (http://findavet.rcvs.org.uk/find-a-vet/) or Vets Now (http://www.vets-now.com/find-an-emergency-vet/) who are open all nights/weekends.
Kind regards *****