My dad gets swayed very easily. How do I help?

My dad has always been swayed very easily with what others suggest or anything new.
Why this worries me..

My dad will come home with this “my client told me about this new kidney cleanse he is on and I think we should do it, you will lose weight as well”
That client was obese and was being monitored by his GP, all throughout the time.

Plus if I am not wrong doesn’t the kidney and liver themselves work together to filter / process toxins, but he was vehemently against being told that this “cleanse” is not for everyone and not even necessary for everyone.
Then few weeks later, he forgot all about it..

This happens quite often, however, this time he is caught in this multi-tiered marketing scheme / pyramid scheme, he was roped in by one of his colleagues and he is so enamoured by their products that he is replacing everything and I mean EVERYTHING with their products.
Soap, bodywash, shampoo.. also is drinking this protein shake that we have no idea what is in it ??

Today he replaced the toothpaste and when I checked that it doesn’t have any fluoride in it, I suggested that we ask our dentist whether it is okay to switch to this? And he got oddly defensive, almost as if I was out here insulting him?

I told him, that it’s a good thing to be a little skeptical and do your own research before jumping head first into things.
But he started defending this company as if it’s his own?
“You don’t know how long this company has lasted” “They are the best at everything” “So what if we have to buy somethings to sell it”

Am I overreacting? This is fairly common experience with my dad, getting carried away with anything new.

Any social media conspiracy post and he immediately believes it to the point, that if I challenge it (as in ask questions) he gets totally defensive.

Recently he was watching an AI video and he completely believed it, when I pointed out that it is AI and even gave him reasons why, pointing out weird stuff in the video and the background, he was still convinced that it isn’t AI.

And no, it’s not something new, when I was barely 2years old, he again got roped into selling this “special” mattress and lost a LOT of money.

This gets very frustrating. Please help me as to how I can help him? Give me guidance that doesn’t make it so I have to argue and break my head with him please, it ends up negatively affecting my mental health as well.