That's interesting , thanks for that. I suspect that if it's an allergy it may most likely be a flea allergy (I think)
The line down the leg I thought was weird and it sounds like what you described. I also read that sometimes a thyroid problem will make them lose hair in a line like that.
I will look into the allergy subject more and go back to Iodine in the water and an Enzyme bath (for fleas and skin mites) and if it nothing works I'll have to take him in to get tested (his thyroid too) and see what it is.