try interceptor. it is in a tablet form, that is supposed to be flavored so dogs will eat them, but if they wont it is shaped like a pill, so you can just pop it down his throat if you have to. have your vet show you how to "pill" him. heres how i do it: lick the end of your index finger on your dominant hand, and get the pill to stick to it. then pry his mouth open, and stick the pill as far back in his throat as you can. you will feel him start swallowing. if it doesnt go down then, just get it off the end of your finger (sttill far back in his mouth), then hold his mouth shut with his nose pointed in the air and rub his throat to make him swallow. good luck.

anothr alternative is the 6 month injection proheart 6, although it has recieved alot of bad reviews on this forum. to date our clinic has not had any dogs show a reaction to it, but admittedly we dont use it very often.