County jail meals are not going to impress anyone. A plate of rice, peas, and a roll is a real meal by institutional standards, even if it would not pass muster anywhere else. Jails operate on tight budgets and nutrition is managed to minimum standards, not maximum enjoyment. Meat appears on the menu with some regularity but not at every meal, and there will be days that feel very lean. That is the reality of county jail food across the country and not a sign of specific mistreatment.
On the broader treatment question, inmates do have legal rights to humane treatment. The Eighth Amendment prohibits cruel and unusual punishment, and if there is genuine abuse, excessive force, or conditions that rise above discomfort into actual mistreatment, that is worth documenting and addressing. Your son can file a grievance through the facility's internal process, and if that produces no results, you can contact the county's jail oversight board or a civil rights attorney.
That said, discomfort and strict treatment are not the same as mistreatment. County jails are not designed to be comfortable, and three weeks inside is going to feel harder than it looks from the outside. The rules are strict, the environment is controlled, and there is very little autonomy. That is the nature of incarceration, not a sign that something is specifically wrong.
Keep the lines of communication open through letters and calls, and take specific concerns seriously while also calibrating expectations appropriately.