Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
Welcome to my forum.
Here you will find more than 6 years of questions and my answers. It is searchable and offers a great deal of information.
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All the best,
Dr. Getty
Hi Lori,
Sounds like you've already made great strides with him -- congratulations! 
I would suggest two things. First, treat him with a five day, double dose of fenbendazole (Panacur Power-Pac) to get rid of encysted larvae. This will get rid of the worms that the other wormers cannot touch. You have to administer the paste five days in a row.
The second suggestion is Glanzen. This is a flaxseed meal based feed/supplement that will bring out the best in him. It is high in B vitamins, Vitamin E, and omega 3 fatty acids. (Don't get the Glanzen-SE, since Wyoming has plenty of Selenium in its soil.)
Please let me know if you have any more questions.
All the best,
Dr. Getty 
Where are you from? Colorado