Thank you for your quesiton, I'm Dr. Dan and I'll try to help you.
What you are describing is an allergic reaction causing a minor infection on the inside of the legs. The crusts you are seeing are likely burst pustules (pimples) that have dried up. The allergy and minor skin infection can be quite itchy which is the likely reason he is dragging himself along. The first thing you can do is pick up a medicated shampoo at the pet store and bathe him now (the crusty area is the only thing that needs to be bathed), and bathe again in 48 hours. If this hasn't made a difference then you need to have him see the vet to get prescription antibiotics and that will fix him right up. I hope this helps, if not please feel free to ask more questions, if it did help, please leave a rating. Dr. Dan