Yeah, please please please please please keep it off of him. We almost lost Socks to a flea collar (it was of a certain brand, that I'm not sure if I can name for legal reasons, but a common over the counter brand). Anyway, she got massive huge scabs and welts and lost a good portion of her fur over the couple days she was wearing it. I felt so bad that I had hurt my baby and we were thisclose to having to get her euthanized because the reaction was so bad and the vet wasn't sure if she would make it (she was just a kitten). I'm not sure if that is common, but it happened to us.
Thank you for taking it off of him.