I am not a big fan of the Powerpac due to the fact that it has been shown to cause severe GI inflammation and even ulceration after use. It's claim to fame is that it kills encysted strongyles, however these aren't what cause weight loss/hard keeping. You would do better off to focus on the whole picture---diet, a good overall deworming plan, possible health issues.
If you want to deworm for encysted strongyles, Moxidectin is also effective without causing the GI inflammation.
How old is your horse? What all does he get fed and in what amounts? What kind of deworming program is he currently on? How is he kept? (Pasture, paddock, dry lot, with other horses, feces picked up daily or every couple of days in paddock/pasture, weather conditions).
Licensed Veterinary Technician