Are they spots or scabs?
Did he have fleas before you bought the collar?
I am just thinking it could be a flea allergy.
If it's fleas it is best to get some spot on stuff from the vets they work so much better than the stuff you get from the shops.
One of mine had a flea allergy a few weeks ago and i got something from the vets and it's now cleared up. It was called Effipro you can get them on line as well and they are a lot cheaper.

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