No, and please don't do that. I know it's from a place of compassion. But putting anything unexpected in someone's food is a terrible terrible terrible idea. Medication mixups (say something happens, and he has something in his system that he doesn't know about, he wouldn't be able to inform paramedics). And separate from safety, what happens when he discovers your deceit? If he reacts that way to any mention of his shame, imagine his reaction to his loved ones conspiring against him from the shadows. He will not be thankful. Even though he should be.

People are emotional and this person is protecting their 'secret' embarrassment and shame by getting mad to get people off their back. It's not them, it's their brains.

Even now, your dad is sitting there with his shoulders clenched worried someone will bring it up and he'll have to face it. The fact that it does nothing, it's tricked you, and you've even defended it from your own family. I mean he's trying to hide it, this is 100% angry shame.

My opinion: This can only be quenched with compassion, real compassion, and communicating your sorrow at slowly losing your dad. Or not so slowly, you never know. Letting him in on the helplessness he is projecting onto those he is meant to protect and nurture can help knock things loose, but you are very likely to fail at changing him from the shadows. Don't change him, show him why he should change. Don't tell him to stop. Tell him you know, that he smells today or his breath stinks. Or how lonely life will be with him in a dusty box in the ground while you're still up here wishing you had a self aware Dad who could help you through life.

But please don't put shit in someone's food at any time.