I would not start with a Powerpac (or generally ever recommend one). If you don't know your horse's deworming history, you should still take a fecal sample to your veterinarian to determine if deworming is necessary. For an adult horse, if deworming is necessary, you should typically use something other than fenbendazole (active ingredient in Powerpac) as that drug has a wide ranging resistance issues with small strongyles which are the main parasite of concern in adult horses.
As for introducing dewormer to your horse, make the attempt as you would with any horse. Halter him, slip the tip of your finger into the edge of his mouth and then follow it with the deworming syringe. If he acts afraid, start by touching and rubbing him with the tube on his shoulder. If he pulls away, keep the tube against him until he stands still, then take it away. Repeat the process until he will stand calmly for the tube being placed against his shoulder, then move up his neck a bit. Repeat this at a couple of spots along his neck and then move to his jaw and finally just inserting the tip into his mouth. At each location, hold the tube against him until he stands still for it before you take it away. Move to a new location closer to his mouth when he is comfortable with being touched with the tube farther away from him mouth. Once you get to the point where you can insert it into his mouth, don't immediately give the medication. First just slide it into his mouth and take it back out, offering a treat afterwards. Do that a few times before you administer the medication. Once you go through this procedure once, repeat it for at least 2-3 more days in a row just to reinforce the lesson. Then you will still need to occasionally stick a tube in his mouth sometimes just to keep him used to it.
Licensed Veterinary Technician