@Aster I empathize with your frustration. Short answer: fear.
Long answer:
sorry, I’m better at long answers…
It sounds like you care very much about your own, and your s/o’s health; but you feel like your s/o doesn’t. Also, your s/o’s reaction to your methods might feel like a personal judgment, shifting from “snake oil is useless” to “anyone who uses snake oil is stupid” (for example), when you have first-hand experience that says otherwise.
To me, it doesn’t sound so much as an “I’m right, but he won’t believe me” problem, but an “I care and want to help, but he won’t allow it” type. This is very frustrating situation to overcome. In my personal experience, it at least takes time. Especially with s/o’s or anyone close to us, we can tout the benefits of anything, but it won’t sink in until it is validated elsewhere, or they “stumble upon” the idea themselves. (I don’t understand why this is, just that it seems to happen a LOT.)
The concept that seems to best fit from my perspective is that people don’t like being wrong, and they don’t like being told that the things they are familiar with, brought up to believe, or otherwise decided they would do, are “wrong”. It’s possible that your s/o thinks that if you’re right, then he’s been wrong, as well as anyone who has lived they way he lives, and well… that’s not possible, so you must be wrong. It’s the only logic that works in their mind because being open to your ideas threatens his identity.
My advice is:
Try to understand/remember that people’s personalities can be so fundamentally different that it can be sometimes impossible to understand someone’s particular behavior, but that’s OK.
Realize/remember that each person evolves through time, and this particular behavior may or may not continue. Internal and external forces change us constantly. In other words, let go of the responsibility you feel to educate him on his health.
Express your care for his health by letting him know that his health is important to you, but respect his own decisions. One can only learn when they are open to it.
Inform him (if this is true) that when he expresses such disdain, that it hurts your feelings. Let him know that he can disagree with you, but there’s no reason why he can’t treat you with respect when you treat his decisions with respect.
When you want to share your excitement about your health successes, focus on the the effects you are experiencing as opposed to what you think he shouldn’t be doing, or your methods. Tell him how much more energy you have, how you’re sleeping better, how you have better digestion or less pain… whatever it is. Besides, THOSE are the things that you want for him – not to stop doing what he enjoys (eating chips), or subscribe to a particular method of health/healing. Demonstrate the benefits of your lifestyle, and be patient so he may get there on his own.
hope this helps