You really shouldn't give a paste dewormer in your horses feed due to the risk that he will not get a full dose---it gets on the sides of the feeder, he won't eat all of the feed with the dewormer on it, etc. Improper dosing of dewormers is a HUGE risk factor in the development of parasites that are resistant to the drugs that we have to use to deworm our horses. Resistance issues are already a big problem with 2 of the 4 drugs in use having moderate to very high incidence of strongyles being resistant to them and with strongyles in some areas starting to show resistance to ivermectin. We simply don't need to take the risk of under-dosing the products we have because resistance will develop more quickly and there are no new deworming drugs set to hit the market--what we have is what we've got.
Rather, you need to focus on training your horse to accept deworming as you would train him to accept leading, saddling and riding. Horses don't just automatically stand to be dewormed, but even the most fearful or bull-headed can learn to. You start by getting them comfortable with the deworming syringe--bring one out regularly and rub it all over your horse so that it's no longer a new and "scary" object. Then start sliding your finger into the corner of his mouth and follow it with they syringe. If he stands calmly, remove it and pet him. If he fights, lunge him quickly and do several direction changes to get him focused on listening to you. Then try to insert the syringe into his mouth again. Anytime he stands, let him stand calmly and pet him but if he fights make him move out and listen to you. And this is not about wearing your horse out, but rather getting him to pay attention to what you want. You can lunge him 1-2 circles, then ask him to change directions 2-3 times after he's gone about 1/2 a circle. It's not about getting him tired, but getting him to pay attention to you and what you want. I have used this method successfully on a very bossy mare who didn't think she should have to pay attention to what people wanted as well as a gelding who is afraid of his own shadow.
The other thing you can do is get away from the old "just deworm every 6-8 weeks" deworming program because it's not a good one. Many horses don't need deworming more than twice a year and 4 times a year would be the maximum necessary. During the time of year when you have the most moderate weather, run a fecal egg count on your horse at least 3 months after using ivermectin or 4 months after using moxidectin and just see what kind of parasite load he normally carries. Each horse develops some level of resistance to parasites and once you determine if yours needs minimal or more deworming you can set up an appropriate deworming program that does the job with the least amount of deworming.
Licensed Veterinary Technician