If it were a reaction to the solutions wouldn't it be all over rather than just on the legs?
As mentioned could be flea bites or at the moment we are inundated with those tiny black thunderbugs, which can make you itch and scratching will bring up the blotches. Another possibilty could just be a heat reaction.
Have you also thought of the possibilty of what is already in/on the carpets before you start could be something that doesn't like his skin?
I would have thought that liquid pre sprays would be worse if he is allergic to the solutions
It makes no sense that it is just on his calfs, what is he doing? Rolling up his trousers and rubbing it on?