It's most likely he has caught something that has fleas (mouse, rabbit) and when he has killed it they have jumped onto him. It doesn't mean Bob Martin is no good, but rabbit fleas especially do seem to like cats ears, and if it's since yesterday the Bod Martin hasn't had any time to take effect. You can get a front line spray, which would be good to spray directly onto his ears.
Our old ginger tom used to get it quite often, and to be honest you can pick the fleas off their ears, if he will let you. Best way to dispose of the fleas is to put them into a wee plastic tub which has water in it, with a little bit of some spray cleaner- flash anything like that will kill them. I once picked over 60 fleas off our old tom's ears in one go.
Hope that helps!
Beth