Lots of good answers here, but I have a slightly different interpretation. This may be something that catches some flack, but reserve judgement until I am done. Maybe your kid is actually a genius. Like a bonified genius, no sarcasm. He sounds a lot like I did when I was his age and it turned out that I am actually somewhere in the top 0.03% of the population. This is not a brag, having this means I have a lot of unique mental health and social issues I have to deal with.

What leads me to the thought are the specific examples he gave. Fundamentally, all of the observations are 100% true.

• Snow isn’t white. Snow is clear, but the way light reflects internally when you stack it up causes nearly 100% of white light to be reflected, which is what makes it look white.
• Ice cream is not cold, as cold is merely a word that describes an absence of heat. The concept of cold is 100% subjective and reliant on a comparison. In essence, you are having a problem with your child’s opinion, which you complained about him having issues with yours. Until you get down below about 30°K, you can’t really say something is objectively cold, as that os where the laws of physics start changing (condensed matter, superconductivity, etc.).
• “It’s not dark yet.” this one is fun on two sides. First, there is a scientifically proven link between what we are taught as a concept of a word and how our brain actually interprets sensory data related to that word. There are societies who do not have words for particular colors, and when tested to identify a differently colored box amongst several boxes with a close hue, they are functionally incapable of discerning the difference. After being taught words to describe the colors and shown examples, they saw a marked improvement in identification. What this can functionally mean is that what you actually perceive as a light level that relates to “being dark” can affect how much of the light that enters your eyes is interpreted as “dark”. Your child does not have this limitation, so his threshold may be much lower than yours. This leads to the second interesting point, it is nearly impossible to create a space that can be occupied that is actually devoid of photons bouncing around. This fact, coupled with your child’s higher sensitivity to light, means that for them, it is actually not dark yet. You are seeing the world through an adult’s lens which says “it is night, so it is dark” instead of the child’s lens that says “dark means there isn’t light, I see light, so it is not dark.”

Even the issues they have with what is known as “sense of other” is an indication. Many geniuses struggle with the concept that other people do not share an identical physical, emotional, or mental condition with themselves. When he says “You’re not hungry” he is really saying “I’m not hungry, so that means you must not be either.” If they are a genius and has that affect, then this will be an uphill battle. Patience and getting them to understand that other people have their own separate thoughts and identities that are vastly different to the point of being incomparable will feel very surreal. It will also have the knock-on effect of making it very hard to directly relate to neurotypical people. My wife and I struggle constantly because I have such a different view of the world than anyone she has ever known.

The minimum age to be tested by Mensa is 4, so it might be worth reaching out to your local chapter and inquiring.