OK I've got it.
Below 12 pounds (dog's body weight) you would need to give pediatric Benadryl.
Otherwise the dose is 1milligram per pound of the dog's body weight.
for instance, a 12 pound dog can take 1/2 a tablet of the Human adult Benadryl (25mg.) tablet.
This only helps the symptoms of course. As for the cause, I am not sure. Maybe someone else with a good idea will post?
It could be something environmental, something he walks in or on when he goes out. Are there any chemicals, even on grass or anything that you think might have got onto his feet? If you take him into countryside, farmers do often spray chemicals onto their land. The problem is, apart from the skin irritation, there is a possibility that if he has got any chemical irritant on his feet, he might ingest some of the stuff too if he bites at his feet. If he is biting at them a lot, and you think it is making the whole thing worse, you may have to put an Elizabethan Collar on him, to prevent that, and to allow the skin on his feet to heal.
Maybe you meant that you clean his feet with antiseptics....? But I wouldn't use any detergents of any kind for this. That could make things worse. Benadryl for the itching might help, because it will prevent him biting at his feet, and give them chance to heal. I have forgotten now, what the dose per pound of body weight is. I'll check it and post again.