I've used neem oil for mites and it works well! I mix 1 teaspoon of neem oil in a 1 pint spray bottle with water. Make sure to use warm water though so the oil doesn't separate. I spray around the vent area. The neem oil oils down the feathers and makes it harder for the mites to lay eggs.
DE will not work. Can you post some pictures of your rooster legs to confirm it's SLM? Here's a good article on how to treat SLM->
https://www.backyardchickens.com/articles/treating-scaly-leg-mites-slm-what-you-need-to-know.77376/