It is normal to get sick once a year and, in fact, healthy because it helps train your immune system.

This is bullshit on many levels.

First, people tend to get sick more than once per year. There are already the flu and common cold and people often get both once per year.

Second, the "once per year" logic is also reminiscent of "seasonality" logic, logic that COVID doesn't really follow. Generally speaking, the common cold, the flu, etc follow cycles where for a given country, there is a surge once per year. This does not apply to COVID, which has had at least two waves per year, often 3 or 4. You can get also COVID 2-3 times per year, as immunity for wanes on the order of 3-6 months (due to its fast evolution, something facilitated by high infection rates).

Third, the idea that getting sick is good for your immune system as an adult is a myth. Pure myth. It has zero scientific support. You should get vaccinations and take reasonable precautions, not aim for getting sick.

It is good for kids to be exposed to various things, including some that may be infectious, buy exactly which ones is pretty murky. When we are little, our immune systems are still figuring out the difference between "self" and "other". If the immune system is inaccurate in this, you may develop an autoimmune disorder, where your immune system attacks "self" because it recognizes it as "other". There is evidence that very young kids playing outside helps avoid this - and similarly, allergies - but it's not a simple thing to disentangle. Either way, it does not apply to your situation.

Covid doesn't cause an impact on your immue system either.

This is a lie. It is well-established that COVID infection causes immune deregulation, and particularly T cell exhaustion. There is more to it, but it effectively "ages" that part of your immune system. This is a very likely contributing factor in why there are so many infectious diseases currently on the uptick. The population, having, on average, been infected by COVID multiple times, has likely taken on a decent amount of immune system damage.

Sometimes you can be In a bit of a state of inflammation which happens in many viral infections.

The main danger of acute COVID infection is massive levels of inflammation mediated by what is known as a cytokine cascade (or storm). It's not "a bit", it's enough to possibly kill you, and the amount more or less makes or breaks how you will fare during the acute phase. This is one of the things that vaccines help with, they help your immune system respond early and keep this modulated.

COVID is also unique in that it infects basically all of your organs and can stay there, causing increased inflammation for months to years. This is not what other viruses do, generally speaking.

And long covid is a topic in itself. It has never been quantified by any and all testing except subjective

A lie. There are many, many studies on Long COVID that quantify it and study it and its etiology.

Hete is a recent one that also describes immune dysregulagulation associated with it. It states that at least 10% of acute COVID infections result in Long COVID. Long COVID is persistent symptoms after acute COVID infection and appears to be, in reality, a continued infection by COVID that just isn't going away and is instead hanging out in various organs, constantly challenging your immune system.

https://www.nature.com/articles/s41590-023-01724-6

maybe he would be more receptive with super libbed up sources or something, i dunno what he would respect

I would not treat him as the primary audience. He is already arrogant enough to spread bullshit despite not understanding the basics of this topic that any medical professional should be expected to know. I would instead present an unassailable case to a wider audience that includes him but is not only him so that peer pressure does its thing and he feels he cannot continue pushing back without looking foolish. Focus on the disease itself and the science. Memorize the numbers. 10% develop long COVIF. Memorize the facts. Long COVIF is medical symptoms that persist after acute COVID infection and there is strong evidence that they come from a continued lower-level infection. T-cell dysregulation and exhaustion is even more common than long COVID. In addition, these numbers represent people's experiences after having COVID and you can be reinfected every 3-6 months, so it's not like only 10% of the population will be impacted. Every infection is rolling the dice. If they were fully independent (which they probably aren't, it's probably somewhere in between), then there is a 10% chance of developing long COVID over your lifetime if you are infected once, 19% if you get it twice, 27% if you get it thrice, 35% if you get it 4 times, etc. The equation to calculate this is 100% * (1 - (0.9^n)), where n is the number of infections. Without precautions, you will likely get COVID 1-3 times per year.

The main risk of doing this in a larger group is that you will need to understand this stuff pretty well, as otherwise he will probably try to appeal to his own authority and various correct-soundkng factoids like he already has. You'll need papers ready to go and cite. You'll have to ask him if he read papers from the Roan or Iwasaki labs. He definitely hasn't lol. If you think he can really change his tune based on a personal conversation, this is better because it gives him a chance to save face. Buy I would be surprised given that he is already happy to spread bullshit like this.