Just an observation (not meant to cause alarm) But sometimes the fleas are great at hiding. I found fleas on my foster cat Chevy when I decided to give him a bath on Friday. I have been using a flea comb on him and to top it all off I am a professional groomer, so I look for fleas every day on dozens of animals, and I did not see them on him until he was wet. So keep looking.

2 other things to consider.

1)If it was just resently he might just be scrating at old bites. A flea bite is much like a mosquito bite. The insect does not cause the itch, the bite does. And it last for days.
2) if Benji is allergic to flea bites, one bite or one flea alon could cause him to scratch.

best of luck with them.