Get Perfect Food in caplet form so he doesn't have to taste it, just has to swallow it. :)
Wait until he's ready to change, to do something about his health. right now it sounds like a rebellion thing, and also bonding thing with his dad - he wants to be like his dad, hasn't laerned that he can be strong and different from his dad.
In matters that require timing and free will, the best thing to do is to leave the subject entirely alone, otherwise, with repeated selling attempts, the other person just digs their heels in in order to assert their right of independent choice. Can you remember any times that your parents told you not to, or to do, something, and even tho you knew they were right, you did differently just because you knew it wasn't waht they wanted? and you didnt' want to have to hear "I told you so?" :)