My son wants to leave his home country and try out a new life. He is only 22. How will he have health insurance?

Difficult issue. You are asking a bunch of USA based docs about how a kid can get insurance in a foreign country. Not something we know much about. Some insurers provide traveler insurance or re-repatriation plans that assist you in returning to the US if you are overseas, and these are found through travel agents. The kid needs to look into the type of access to healthcare available where he plans to live.
Many options. The easiest way to get insurance is through an employer. So he should look for a job that offers health insurance. There is also medicaid - health insurance for the very poor. On the private market, he can elect to purchase health insurance, but premiums may be very high.