Safety in prison is largely within your son's own control, and that is actually a more reassuring answer than it might sound at first.
The inmates who have the most difficult time inside are generally the ones who create problems, get involved in other people's business, or carry attitudes that invite conflict. Someone who keeps to themselves, follows the rules without attitude, does their work assignment, and uses their social skills to navigate relationships will move through a sentence with far less friction than someone who does not. The same instincts that help people get along on the outside, reading a room, picking your battles, and treating people with basic respect, apply inside too.
On the money question, not having funds on the books is not ideal but it is also not a disaster. Most inmates are required to work, and that work assignment provides a small but real income. Prison wages are minimal, often just a few cents to a few dollars per day depending on the facility and the assignment, but over time it accumulates. A steady work assignment also provides something arguably more valuable than the pay, which is structure, routine, and a sense of purpose during the day. That routine is one of the most stabilizing things inside.
The work income can cover basic commissary needs over time. It will not cover everything, but it keeps him from being entirely dependent on outside support. Facilities are also required to provide indigent inmates with necessities, including hygiene items and letter-writing materials, so the baseline is covered.
When you can put a little something on his books, even a small amount makes a meaningful difference. But it is not a requirement for him to be okay.