For most of our worming, we use a product with DE and herbs, which works great. We get it from Advanced Biological Concepts (website is www.a-b-c-plus.com
). I'd use double their normal dosing until you get the problem under control. It's not cheap, but it is cheaper than ivermectin (as well as being non-toxic). Besides that ... perhaps making a mix of essential oils in a base of olive oil (or some other cheap oil), and spreading it on his topline after a bath, to both sooth the skin and repel insects. There's been discussion of tea tree oil in other forums -- I don't know if it repels insects, but it would probably help heal the sores.