So the active ingredient is Deltamethrin. This is a synthetic pyrethroid similar permethrin which undergoes rapid biotransformation by liver enzymes, which limit their systemic toxicity.
So... it is likely that your dog will be fine. The liver will clear the toxin before it has an effect, but it depends somewhat on how much deltamethrin was in the collar, how big your dog is, and how efficient his liver is. And for many of these factors there is no way to know.... was the collar new? If it was new I'd be more concerned. If your dog is small I'd be more concerned.
Signs of toxicosis include salivation, vomiting ypersalivation, hyperexcitability, tremors, ataxia (loss of balance), and weakness. Dogs more frequently exhibit mild to moderate tremors and ataxia.
If you wanted to be on the safe side, you could take him to a veterinarian to get some activated charcoal to help bind any toxin left in the intestine and to have him monitored for symptoms. If not, then just watch close and have them looked at if he has any signs of toxicity. Most of the time they will be fine, but there are so many factors at play that we can't be sure.
The other concern is that the collar could present a risk of blockage. This would depend on how well he chewed it and if there were any large pieces swallowed. If you see vomiting, definitely get him checked out.
I hope that helps, but if you have more questions please let me know.